How to Delete an iTunes Account on an iPhone
The iPhone uses the account information for the iTunes account that is currently signed in on the device. To delete or remove an iTunes account from an iPhone, access "Store" in the Settings panel. Choose to sign out to successfully remove the account from the phone. If you wish to sign in with a different iTunes account, tap "Sign In" and then enter the account user name and password to add a new iTunes store account to the iPhone.
Tap the "Settings" icon on the iPhone's main menu.
Tap on the "Store" tab under "Settings."
Tap the "Sign Out" tab to exit the iTunes account and remove it from your iPhone.