How to Switch iTunes Accounts on an iPhone
Buying music and video in the iTunes Store on your iPhone allows you to use the files immediately. Best of all, you can use the same Apple ID account that you use on your computer's iTunes program. Once you log in on your iPhone using an Apple ID, the iTunes application will charge all purchases to that account unless you change it. Switch to a new Apple ID for the iTunes Store at any time on your iPhone.
Tap the "Settings" icon on your iPhone's home screen.
Scroll down to the bottom of the screen and tap "Store."
Tap the name of the current Apple ID, then tap "Sign Out" in the pop-up menu.
Tap "Sign In" and enter the new Apple ID and password.