Tap Music from the bottom navigation bar if you’re in the iTunes Store, then scroll to the bottom of the page. If you’re using the App Store, tap Featured from the bottom navigation bar and scroll to the bottom of the page.
In the text box, enter your 16-digit code which starts with an “X”. (If you have an iTunes Gift Card, this is printed on the back of the card).
When done, tap Redeem.
If you are not already signed in, you will be prompted to do so after tapping Redeem. Once you have successfully redeemed your iTunes Gift Card, you will see the following Thank You page.