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In the help where everything else is; add an upgrade button. This simply needs to be a link to the latest version online.
If not in Help then how about another button in the About window?
With the Jet 2019 release (19.0.18268.1), a notification button is added to the Jet tab if the add-in version is less than the version of Jet Service Tier it is connected to. If not connected to a Jet Service Tier, then it checks for the version available for download.
Read more about it in the What's New or Release Notes articles.
This is probably about the Excel add-in? The upgrade should be limited to the same version the web portal or the service tier have, right? If you do this, a version number comparison with a popup to mention the available update would be awesome...