method one
1. Open the App Store, click on the button in the upper right corner of the picture (picture search interface no buttons);
2. Select the "Redeem a gift card or code" option;
3. Enter the redemption code to App you get.
Method 2
1. Open the bottom of the "App" and "games" in the App Store Tab page;
2. Slide the bottom of the page, click on "Redeem" button;
3. Enter the redemption code to get you in the next screen.
If your iPhone / iPad version lower phone system can freely explore redeem it. Exchange is easy to find the entrance.