How do I get back a lost PG3D account using Game Center on IOS?

My brother had a problem where he accidentally switched his account in the game pixel gun 3d. He uses Game Center on iOS 14 iPad. He switched it to his noob account, and he can’t switch it back because the “switch account progress” option won’t pop up. It will appear with two icons, one with a mutant (his pro account) and one with a soldier (his noob account). Currently he is in his noob account. So how do I help him change his account back? If you don’t know, maybe someone can tell me how to make the “switch account progress” option pop up again.