Q: I cannot use iCloud since the update

Since the IOS7 update I cannot use iCloud as the account has gone back to my old email address.  Can anyone tell me how I can change the account details; I don't want to change my Apple ID - I know how to do that and also change the password - I want to know how to change the iCloud account from my old email address to my present one.  I don't know how to do it and I can't seem to find any details of how to do it.  Any suggestions please?  Thanks.