what i do is, every start the app download a txt file (if net connection available)
in this file i write the app version
now i have put an button in the app with version information (button color, light blue)
if i want to inform my users about a new update i change the txt in the file to the latest version
now, if txt <> oldversionint then (button image change to red (and if you click on it msg box new update available))
now if i want all user to update i put also this condition to my app
if txt = "x" then msgbox("A new critical update is available...(bla,bla,bla)","") and app close
of course every update i change the condition (because if not it will close after update too) so in the new build i put another letter then the last build (if it was "x" in the new it will be "y"....)
*** (in short explaination)
1. put a button with an image with default color and inform your user for a new update by change the image to red...
2. put an possibility to force the app to close if you want user to update immidiatly
you better use 2 lines in your txt file (1st line for the App VersionCode, 2nd line "x" to force closing)
Good luck