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You can find information about updating your payment methods registered for your AppleID account here:
Cancel or change the payment method for your AppleCare plan - Apple Support
It is often a good idea to have two (or more) payment cards associated with your account - as, in case there is a problem with your primary card, a charge can be made to the second card without risk of impacting ongoing access to your AppleID account or associated services and subscriptions. With multiple Credit Cards, you can choose the sequence in which card payments are made.
You may find these support pages to be helpful:
If you need to change or update your Apple ID payment method - Apple Support
Add a payment method to your Apple ID - Apple Support
Remove a payment method from your Apple ID - Apple Support
Payment methods that you can use with your Apple ID - Apple Support
If you have any issues, you would be best advised to seek assistance directly from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. The App also provides direct access to appropriate support resources. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044