if they sync with Garmin Express (and are set to auto-update apps) they'll get the new version when they sync. If they use Garmin Connect Mobile for iOS, there is an indication that an update is available, but with GCM/Android, there is no sign of an update. There's no way to send out a message to the users.
Make sure in the app store you not an update in the "what's new" part of the description, and what may help is having a CIQ showcase thread for your app. Add a post there when you do an update (I put more details in the post and in "what's new" in the store, I provide a link to the post)
Other than that, about all you can do is ask the user to install the latest version if they are having a problem.
