See -> Add a payment method to your Apple ID
Add a payment method to your Apple ID – Apple Support (UK)
- If you want to change your card, you need to add first the debit card, then remove the credit card.
See also -> Payment methods you can use with your Apple ID
Payment methods you can use with your Apple ID – Apple Support (UK)
fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.
If you update your payment method or add funds to your Apple Account balance, It would take about 1 to 3 business days in order to process.
If you're still unable to update your payment method, Contact Apple Support.