You don't have to change anything. Redeem the cards to your account if you have not already. They will automatically be used for payment if it is something for which account balance can be used as payment. If you get a message telling you that you need to add a payment method then apparently you won't be able to use them alone and will need to add a card again.
The article "What type of gift card do I have?" explains the different kinds of gift cards Apple uses, has links on how to redeem them, and what can be purchased with each kind. Click on the link --> https://support.apple.com/HT204199
See this article about how to add a payment method to your Apple Account --> https://support.apple.com/HT201266