You can change your payment method in your account settings.

1. Open the App Store on your Mac.
2. Click your name or the sign-in button at the bottom of the sidebar.
3. Click View Information.
4. Next to Payment Information, click Manage Payments. (If you're using an older version of macOS, click Edit to the right of Payment Type.) You might be asked to sign in with your Apple ID.
5. From here, you can do these things:
   a. Update a payment method—Click Edit next to the payment method. You might need to enter the security code that's printed on your credit or debit card.
   b. Add a payment method—Click Add Payment.
   c. Remove a payment method—Click Edit next to the payment method, then click Remove.
   d. Change the order of your payment methods—Hover your pointer over a payment method, then use the move order icon. Apple will attempt to charge payment methods in order from top to bottom. You can't change the order in which Apple ID balance is charged.

📌 If you're still having problems making purchases, please contact your application store's customer support.