Please follow the instructions below to redeem your code:
Step 1. Open the iTunes app store on your device and make sure that you’re on the featured screen.
Step 2. Scroll to the bottom of the screen until you see the buttons for; Redeem, Send Gift, Apple ID.
Then click Redeem.
Step 3. You’ll be prompted to sign in to the iTunes store.
Step 4. Once you're signed in then you’ll see the screen where you can enter the promo code. Click You can also enter your code manually.
Step 5. Enter Promo Code, where it says You can also enter your code manually above the keyboard.