I need to change the apple ID used for icloud

From http://support.apple.com/kb/HT5621

To change your Apple ID, follow these steps:

1. Go to My Apple ID (appleid.apple.com), click "Manage your Apple ID", and sign in.
2. If you have two-step verification turned on, you'll be asked to send a verification code to the trusted device associated with your Apple ID. If you are unable to receive messages at your trusted device, follow the guidelines for what to do if you can't sign in with two-step verification.
3. In the "Apple ID and Primary Email Address section," click Edit.
4. Enter the email address you want to use, then click Save Changes. Apple will send a verification email to that address.
5. Open the email from Apple, then click Verify Now in the email.
6. When the My Apple ID page opens, sign in with your renamed Apple ID.
7. If you have two-step verification turned on, you'll be asked to send a verification code to the trusted device associated with your Apple ID.

After you see a message indicating that verification is complete, remember to update all of the features and services that you use with your Apple ID.

If you need to merge accounts you have to call support. They can't do that in a store or online.