Change payment method
How to change payment using gift card not credit or debit
Question marked as
If you add a gift card to your Apple ID, purchase will always come off of that balance before your credit or debit card is charged. This explains how to redeem a gift card.
Redeem your App Store & iTunes Gift Card - Apple Support
If you want to change or remove a card, this should be helpful.
Change, add, or remove Apple ID payment methods - Apple Support