Cannot update my subscription payment, keeps declining my card payment

Verify your payment information -> Sign in to your Apple ID account page and edit / update your Apple ID details.

https://appleid.apple.com                                                     

• On the Sidebar click Payment methods -> then tap Payments Methods and tap Manage payment method.

Or 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 new card, then remove the old card.

See also -> If your payment method is declined in the App Store or iTunes Store, -> 

If your payment method is declined in the App Store or iTunes Store - Apple Support

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How to complete your purchase if a payment failed or was declined

1. On your iPhone or iPad, open the Settings app.
2. Tap your name.
3. Tap Payment & Shipping.
4. Add a different payment method and remove the old one. 
5. Try to complete your purchase.

If you still can't make a purchase

• You might have an unpaid order. Learn how to pay an unpaid order.
• Contact your financial institution. 
  • You might need to ask your financial institution to allow international purchases on your debit or credit card.
  • If you want to know why a debit card, credit card, or other payment method was declined, speak with your financial institution. Apple doesn't have information about why the payment method was declined.
• In some countries and regions, your payment method might be declined because of changes to local law. Learn about billing and purchases in Europe or what to do if your card is declined in India.
• Still need help? Contact Apple Support.

Click here for more support and assistance.

Use this resource about what to do if your payment method is declined in the App Store or iTunes Store. Click here --> If your payment method is declined in the App Store or iTunes Store - Apple Support

- Apple is checking with your financial institution if the payment method is valid and receiving a negative response. Make sure the name and address are entered exactly as it appears on your financial institution bills and that the 3 digit CVV is correct. Check with your financial institution about the status of your card.

- Make sure your payment method is up to date or to try a different one ( If you need to change or update your Apple ID payment method - Apple Support ).

- Make sure this is one of the payment methods that you can use with your Apple ID ( Payment methods that you can use with your Apple ID - Apple Support ) A solution may be to use a method from the same country as the store you are using, but Apple no longer states either way if this is a requirement (it used to say, "Make sure that the billing address for your payment method matches the billing address for your Apple ID" but has now downsized this to "You might need to ask your financial institution to allow international purchases on your debit or credit card." in: If your payment method is declined in the App Store or iTunes Store - Apple Support .)

・Unfortunately the list is somewhat general and does not list individual brands of cards (I guess there are too many and are constantly changing).

・Also read any footnotes in the document.

・Finally, note that the document only says which methods _are_ acceptable. Apparently certain types (usually a credit or debit card) _must_ also be present under certain situations such as when a subscription is involved.

If you still have problems, contact official Apple Support. Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

See a list of Apple phone numbers around the world, in this article --> Contact Apple for support and service - Apple Support

If you're in India, click on this link to read an April 2022 notice 'About billing for Apple subscriptions and media products in India' --> About billing for Apple subscriptions and media products in India - Apple Support

Additional verification that may be required when making purchases in Europe --> About billing and purchases in Europe - Apple Support