Where to update auto payment?

If this is for a subscription managed by Apple from an in-app purchase, then you just need to add the payment method you want to use and/or change the order of the payment method by tapping the Edit button on that page.

Add a payment method to your Apple Account - Apple Support

If this is an auto payment from a third party, you will need to contact them for any changes.

See this article for information about updating payment methods on various platforms (iPhone, Mac, etc.): If you need to change or update your Apple Account payment method - Apple Support

Also note:

1) ”If you change your address, the credit or debit card information that you use with your Apple Account might be automatically updated by your financial institution, and you don't need to take action.” (More about that here Protecting against fraudulent Apple.com c… - Apple Community )

2) “If there's an issue [such as you having a subscription or being a Family Sharing Organizer], add a new card first, then remove the old card."