To redeem a promo code for an app within the Apple app store please follow the steps below:
Step 1: Open the App store on your device and make sure that you’re on the featured screen. Click on the top right Circle "account" Icon.
Step 2: Click "Redeem Gift Card or Code"
Step 3: You’ll be prompted to sign in to the iTunes store.
Step 4: Once signed in, click "You can also enter your code manually."
Step 5: Enter your Promo Code