How do I Change an iTunes ID on an iPhone?
Apple asks you to list the Apple ID for your iTunes Store account when you first set up your iPhone. This account is associated with the iPhone when you download music from the iTunes Store or applications from the App Store on your iPhone. You can change the Apple ID associated with your iPhone by signing in with it on your computer and doing a basic sync between iTunes and your iPhone.
Tap the "Settings" application on your iPhone's app screen, and select "Store."
Tap the "Apple ID" window and tap "Sign Out" in the appearing dialog box.
Tap "Sign In," "Use Existing Apple ID." Enter the information for the new Apple ID you want to use in the given fields and tap "OK."