Android - Is there a simple way to undo an app update?

Android does not natively back up applications so you can't "undo" an app update. Best thing I'd say you can do is something like this howto. It backs up your apks and their settings. It uses Titanium Backup and requires root, but rooting is not your problem.

Unfortunately once the new version is installed there is no way for you to roll back. The only way you can get back to the old one is if you already have a copy of, or can manage to find the APK file for the version that you want.