- From your iPhone, iPad or iPod touch
1. Tap iTunes Store, App Store, or iBooks Store on your device.
2 .Scroll to the bottom of the Featured section and tap Redeem. You will need to sign in with your Apple ID.
3. Tap "You can also enter your code manually." In some countries, you can redeem gift cards using your built-in camera.
4. Enter your unique code displayed on the ‘Orders’ page of your CDKeys account or on the link of your confirmation email if you had purchased as a guest and select 'Redeem'.
5. After you redeem a gift card code, your iTunes account balance will update. You might need to sign out and back in to all of your devices for your balance to appear. You can find your balance underneath your Apple ID.
If you redeemed a content code, your content will begin downloading.
When you redeem a gift card, the amount of the card is credited to your iTunes Store, App Store, iBooks Store, or Mac App Store accounts. Each time you purchase content, iTunes will deduct funds from the credit until it's gone.
- From your Mac or PC, the Mac App Store, or the iBooks Store
1. Depending on where you want to redeem your code, open iTunes, the Mac App Store, or iBooks. (Make sure that you have the latest version of iTunes.)
2. From the menu bar, click Store > Sign In to sign in using your Apple ID, if you're not already signed in.
3. Click 'iTunes Store' in iTunes, Featured in the Mac App Store, or iBooks Store in iBooks.
4. Under the Quick Links section on the right-hand side, click Redeem.
5. Enter your unique code displayed on the ‘Orders’ page of your CDKeys account or on the link of your confirmation email if you had purchased as a guest, then press Return. 
6. If you redeemed a content code, your content, app, or book will automatically begin downloading.
After you redeem a gift card code, your iTunes account balance will update. You might need to sign out and back in for your balance to appear. You can check your balance at any time in your account information.
When you redeem a gift card, the amount of the card is credited to your iTunes Store, App Store, iBooks Store, or Mac App Store accounts. Each time you purchase content, iTunes will deduct funds from the credit until it's gone.