If you run into iOS 12.3.1 problems, you don’t like the changes or you just want to go back to what you’re used to you can downgrade from iOS 12.3.1 to iOS 12.3 for a limited time. While there are generally not many downsides, if you downgrade you may be open to a nasty Group FaceTime bug.

You can downgrade from iOS 12.3.1, but not all the way back to iOS 12 or to iOS 11 at all. You can then restore a backup you made while on iOS 12.3. You cannot restore a backup made on iOS 12.3.1 after you go back.
…
Read full post here:
https://www.gottabemobile.com/how-to-downgrade-from-the-ios-12-to-ios-11/