How do I change my iPhone from one iCloud account to the other?
Tap Settings > iCloud > Delete account. Then setup the account you want.
Apple does not support merging of account > Frequently Asked Questions About Apple ID
ICloud does not work properly until Apple will allow us to merge our accounts. I have this problem too. We actually now have a worse situation than we had without iCloud because in order to transfer content we have to log in and synch for each account separately. In my case the solution provided in the comment, above, does not work because it says I have to verify the second account even though it is already verified according to my desktop machine. It is a mess. Many people are frustrated with it and waiting to be able to merge accounts.