2 Game Center accounts under 1 Apple ID.

I have 2 Game Center accounts,under 1 Apple ID, on. Of which I am logged in to. I would like to change to the other account because the other one has my advanced clash of clans account on it and I do not want to start a new one. How do I change to the other one and also get the password for it because I forgot eh password on the other one but i do know the name. Thanks you whoever is answering for their time.