how to change iCloud debit account
How do I change the iCloud additional storage debit bank account
iCloud uses the payment methods added to your Apple ID, in the order they appear in.
If you want to change the payment method, add a new one to your Apple ID, and drag it to the top of the list so its the first option under your Account Balance. Then it will use that first for all purchases including iCloud as long as there is no Account Balance to use.