If you have promo code for apple iTunes Appstore specific for any paid app, here is how you can redeem that apps with promocode with the help of iOS App Store as given 5 steps.
1) Open App Store app
2)Click on featured and redeem code.
3)Now it will prompt to enter your apple iTune account password. Enter your password and click ok.
4)If you just got a promo code only, click on “You can also enter your code manually”.
5)Now enter you promocode and press return or ok. After that it will auto start to download the app
Hope this quick How to redeem app promo codes in Apple iTunes Appstore helps you. If you got any query or suggestion please drop a comment.