Following is a simple method to check for available updates of Add-ins/Programs:

The procedure can be called on start up of Excel or on displaying an About Dialog-box (as I decided in my case being the less intrusive variant). The parameter doUpdate decides whether only the check for the new version is performed or the new version is actually downloaded. The update check/download requires a continuously increasing version number being available via a URL (here on githubs tag/release archive) of the form https://domain.name/path/1.0.0.<release>. A local update folder can also be provided to allow for a central update by an administrator.

The UserMsg and QuestionMsg are just wrappers of MsgBox providing a headless mode (without pop-ups) and logging so you can replace them with your choice of MsgBox(theMessage, msgboxIcon + questionType, questionTitle).

First, the necessary settings are fetched, you can also hard-code the default values (being the second argument of fetchSetting, described in more detail in User settings and the .xll.config file)

    Public Sub checkForUpdate(doUpdate As Boolean)
        Const updateFilenameZip = "downloadedVersion.zip"
        Dim localUpdateFolder As String = fetchSetting("localUpdateFolder", "")
        Dim localUpdateMessage As String = fetchSetting("localUpdateMessage", "A new version is available in the local update folder, after quitting Excel (is done next) start deployAddin.cmd to install it.")
        Dim updatesMajorVersion As String = fetchSetting("updatesMajorVersion", "1.0.0.")
        Dim updatesDownloadFolder As String = fetchSetting("updatesDownloadFolder", "C:\temp\")

        ' put your UrlBase here, where the release zip files can be found
        Dim updatesUrlBase As String = fetchSetting("updatesUrlBase", "https://github.com/rkapl123/DBAddin/archive/refs/tags/")
        Dim response As Net.HttpWebResponse = Nothing
        Dim urlFile As String = ""

Then, the procedure checks check for the zip file of the next higher revisions until no higher version can be found. Net.SecurityProtocolType.Tls12 is only available starting with .NET 4.5

        Dim curRevision As Integer = My.Application.Info.Version.Revision
        ' try with highest possible Security protocol
        Try
            Net.ServicePointManager.SecurityProtocol = Net.SecurityProtocolType.Tls12 Or Net.SecurityProtocolType.SystemDefault
        Catch ex As Exception
            UserMsg("Error setting the SecurityProtocol: " + ex.Message())
            Exit Sub
        End Try
        ' always accept url certificate as valid
        Net.ServicePointManager.ServerCertificateValidationCallback = AddressOf ValidationCallbackHandler

        Do
            urlFile = updatesUrlBase + updatesMajorVersion + (curRevision + 1).ToString() + ".zip"
            Dim request As Net.HttpWebRequest
            Try
                request = Net.WebRequest.Create(urlFile)
                response = Nothing
                request.Method = "HEAD"
                response = request.GetResponse()
            Catch ex As Exception
            End Try
            If response IsNot Nothing Then
                curRevision += 1
                response.Close()
            End If
        Loop Until response Is Nothing

get out, if no newer version could be found. In my case, I set a TextBox (TextBoxDescription) and a button (CheckForUpdates) to notify the user. for the notification step (doUpdate = False) stop here and let the user decide by clicking on the button CheckForUpdates.

        If curRevision = My.Application.Info.Version.Revision Then
            Me.TextBoxDescription.Text = My.Application.Info.Description + vbCrLf + vbCrLf + "You have the latest version (" + updatesMajorVersion + curRevision.ToString() + ")."
            Me.TextBoxDescription.BackColor = Drawing.Color.FromKnownColor(Drawing.KnownColor.Control)
            Me.CheckForUpdates.Text = "no Update ..."
            Me.CheckForUpdates.Enabled = False
            Me.Refresh()
            Exit Sub
        Else
            Me.TextBoxDescription.Text = My.Application.Info.Description + vbCrLf + vbCrLf + "A new version (" + updatesMajorVersion + curRevision.ToString() + ") is available " + IIf(localUpdateFolder <> "", "in " + localUpdateFolder, "on github")
            Me.TextBoxDescription.BackColor = Drawing.Color.DarkOrange
            Me.CheckForUpdates.Text = "get Update ..."
            Me.CheckForUpdates.Enabled = True
            Me.Refresh()
            If Not doUpdate Then Exit Sub
        End If

If there is a maintained local update folder, open it and let user update from there.

        If localUpdateFolder <> "" Then
            Try
                If QuestionMsg(localUpdateMessage, MsgBoxStyle.OkCancel) = MsgBoxResult.Ok Then
                    System.Diagnostics.Process.Start("explorer.exe", localUpdateFolder)
                    Me.quitExcelAfterwards = True
                    Me.Close()
                End If
            Catch ex As Exception
                UserMsg("Error when opening local update folder: " + ex.Message())
            End Try
            Exit Sub
        End If

Otherwise continue and download newest version. Progress information is put into the TextBoxDescription

        urlFile = updatesUrlBase + updatesMajorVersion + curRevision.ToString() + ".zip"

        ' create the download folder
        Try
            IO.Directory.CreateDirectory(updatesDownloadFolder)
        Catch ex As Exception
            UserMsg("Couldn't create file download folder (" + updatesDownloadFolder + "): " + ex.Message())
            Exit Sub
        End Try

        Me.TextBoxDescription.Text = My.Application.Info.Description + vbCrLf + vbCrLf + "Downloading new version from " + urlFile
        Me.Refresh()
        ' get the new version zip-file
        Dim requestGet As Net.HttpWebRequest = Net.WebRequest.Create(urlFile)
        requestGet.Method = "GET"
        Try
            response = requestGet.GetResponse()
        Catch ex As Exception
            UserMsg("Error when downloading new version: " + ex.Message())
            Exit Sub
        End Try
        ' save the version as zip file
        If response IsNot Nothing Then
            Dim receiveStream As Stream = response.GetResponseStream()
            Using downloadFile As IO.FileStream = File.Create(updatesDownloadFolder + updateFilenameZip)
                receiveStream.CopyTo(downloadFile)
            End Using
        End If
        response.Close()
        Me.TextBoxDescription.Text = My.Application.Info.Description + vbCrLf + vbCrLf + "Extracting " + urlFile + " to " + updatesDownloadFolder
        Me.Refresh()
        ' now extract the downloaded file and open the Distribution folder, first remove any existing folder...
        Try
            Directory.Delete(updatesDownloadFolder + "DBAddin-" + updatesMajorVersion + curRevision.ToString(), True)
        Catch ex As Exception : End Try
        Try
            IO.Compression.ZipFile.ExtractToDirectory(updatesDownloadFolder + updateFilenameZip, updatesDownloadFolder)
        Catch ex As Exception
            UserMsg("Error when extracting new version: " + ex.Message())
        End Try
        Me.TextBoxDescription.Text = My.Application.Info.Description + vbCrLf + vbCrLf + "New version in " + updatesDownloadFolder + "DBAddin-" + updatesMajorVersion + curRevision.ToString() + "\Distribution, start deployAddin.cmd to install the new Version."
        Me.Refresh()

Finally, open the windows explorer to let the user start the update process. After leaving the procedure, a hint to close Excel (or do this automatically) is probably a good idea (the Addin won’t be deployed as long Excel is open)…

        Try
            System.Diagnostics.Process.Start("explorer.exe", updatesDownloadFolder + "DBAddin-" + updatesMajorVersion + curRevision.ToString() + "\Distribution")
        Catch ex As Exception
            UserMsg("Error when opening Distribution folder of new version: " + ex.Message())
        End Try
    End Sub

The ValidationCallbackHandler just returns true, if some more checks are needed put them here.

    Private Function ValidationCallbackHandler() As Boolean
        Return True
    End Function

For information, the fetchSetting function is shown here, for more details see User settings and the .xll.config file

    Public Function fetchSetting(Key As String, defaultValue As String) As String
        Dim UserSettings As Collections.Specialized.NameValueCollection = Nothing
        Dim AddinAppSettings As Collections.Specialized.NameValueCollection = Nothing
        Try : UserSettings = ConfigurationManager.GetSection("UserSettings") : Catch ex As Exception : End Try
        Try : AddinAppSettings = ConfigurationManager.AppSettings : Catch ex As Exception : End Try
        ' user specific settings are in UserSettings section in separate file
        If UserSettings(Key) Is Nothing Then
            If AddinAppSettings IsNot Nothing Then
                fetchSetting = AddinAppSettings(Key)
            Else
                fetchSetting = Nothing
            End If
        Else
            fetchSetting = UserSettings(Key)
        End If
        If fetchSetting Is Nothing Then fetchSetting = defaultValue
    End Function