If you download apps, movies, and games on your iPhone or iPad, you're already familiar with what an iTunes account is and how it works. However, that doesn't always mean that the email or account you use for mail and iCloud is the same account you use for purchases. If you ever need to change what iTunes account is linked to your iPhone or iPad, you can do so in just a few steps.
How to switch iTunes accounts on your iPhone or iPad
Launch the Settings app on your iPhone or iPad.
Tap on iTunes & App Store — you'll need to scroll down a ways.
Tap on your current Apple ID at the top.
Tap on Sign Out in the popup menu that appears.
Sign into a different Apple ID in the fields provided.
That's all there is to it. There are lots of reasons you may need to switch between Apple IDs. Perhaps you have purchases under multiple accounts or an older email, or you have multiple accounts in your household. Whatever the reason, signing into another account you are authorized to use should then show any apps, music, and movies that have already been purchased.