Visit our gift redemption page. This should be done on a computer or your mobile browser (not in the app itself). Once on the web page, you will be prompted to sign in (or create an account if you have not already). Once you have signed in, you can redeem your gift with the activation code. Then all you need to do is log into the app with the same account and the new subscription will be active on your device.
Note: You will be unable to redeem a gift certificate if you currently have an active subscription on your account. You can see if you have an active subscription by following the instructions here.