how to switch between iTunes accounts

Change the Apple ID used with a device - Settings > Store > tap on your AppleID

https://discussions.apple.com/thread/3980210?answerId=18484935022#18484935022 - "In order to change your Apple ID or password for your iCloud account on your iOS device, you need to delete the account from your iOS device first, then add it back using your updated details. (Settings > iCloud, scroll down and hit "Delete Account")

When you delete your account, all the data that is synced with iCloud will also be deleted from the device (but not from iCloud), but will be synced back to your device when you login again."

Start iTunes and immediately hold down the shift key.