If it's an Android phone you can download an older version might get it going, you'd need sign in again.
https://www.apk4fun.com/history/124732/ 
On iOS it can be done but more tricky.
Or just download the latest firmware and be done with it, reversing that is considerably harder though so (almost) no going back.

They have made it harder to revert as they introduced enhanced control to stop people flying in areas such as near airports.
Most won't be affected and obviously you *might* be unaware of small airfields if flying in a less familiar location so mostly not a bad thing.