I found an article with steps you can take to update your billing information on a computer or on an iOS device.
You can update the billing information on your iPad using these steps:
Changing your account information using an iOS device:
- Tap Settings on the Home screen.
- Tap iTunes & App Stores.
- Tap on your Apple ID. (If you are not signed in, enter your Apple ID and password, and tap Sign In.)
- Tap View Apple ID.
- Enter your Apple ID password.
- In the Edit section, tap Payment Information.
- Update the information that you want to change.
- Tap the Done button when finished.
You can find the full article here:
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