John, to log out go to the "Me" tab in the DJI Go app, tap the gear icon and then "Sign out"; you'll be able to sign in to your DJI account again once you're signed out. On the same screen you can also clear the app cache.
It doesn't really seem to always work though, whereas killing the app (double tap on the home button and then swiping up from the recent apps list) *seems* to be a more reliable approach, with or without signing out and back in.