How do I change iCloud account without the password to the one i want to remove?

When I got a new phone to return my cousins phone to him, my icloud transferred over for apps and payments but for the icloud it grabbed his account which he cant remember the password so i cant sign in and sign out. It only affects photos and imessage features but I keep getting popups to sign in. I've deleted the current icloud and put in mine hoping it would reset, verified the phone # and got a message saying I would be contacted at that # with instructions how to reset but I haven't received a call. Also my cousin is being a jerk about it so he is no help. I just want to sign in to my Icloud so i stop getting these pop ups.