How to Authorize an iPhone
In order to download and install applications from Apple's iOS App Store, you must authorize your iPhone with a valid Apple ID. This Apple ID is the same account !D you use to purchase content from the iTunes Store on your Mac or PC computer. You can only be logged in to your iPhone with a single Apple ID at a time, but you can change the authorization account at any time.
Things You'll Need
Tap on the App Store icon from the iPhone's homescreen.
Go to the "Featured" tab along the bottom of the screen.
Scroll to the very bottom of the page and click on the "Account" button.
Choose the "Sign Out" option from the dialog window that appears.
Tap on the "Sign In" button and then select the "Use Existing Account" option.
Enter your Apple ID username and password using the on-screen keyboard. These credentials are the same ones you use to purchase content from the iTunes Store.