If you can’t find a subscription use this shortcut to jump to the section about what to do (ignore the article title). --> If you want to cancel a subscription from Apple - Apple Support If this is a subscription not made through Apple, “you might have bought the subscription from another company. To find out which company bills you, check your bank or credit card statement. To cancel the subscription, you must contact the company that bills you for the subscription.” About purchases not billed through the App Store or the App Store’s In-App Purchase system - Apple Support
If you still can't find a subscription or still have problems, contact Apple Support using this issue-specific link at the end of the section. --> https://getsupport.apple.com/?caller=kbase&PGF=PGF63006&category_id=SC0060&symptom_id=23208
Scroll down to where you can request a chat session or a phone call.
Note, however, that you cannot change payment method for a specific item. It applies to all items unless you rearrange the order in which a payment method appears just before a payment is taken.
From this support article --> How apps, content, and subscriptions from Apple are billed - Apple Support
"Which payment method is charged?
When you buy items or pay for subscriptions, Apple automatically attempts to charge your payment methods in this order:
1. Apple attempts to use your Apple ID balance to cover the full amount.
2. If you don't have Apple ID balance, you don't have enough to cover the full amount, or an item cannot be paid for with Apple ID balance, Apple attempts to charge your primary payment method (typically a credit or debit card, but varies according to country). The primary payment method is at the top of the list in Settings on your iPhone. You can control which payment method is at the top of the list. [See: Add a payment method to your Apple Account - Apple Support ]
3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple may try again."
You cannot designate a specific card for a specific subscription. You would have to change the card order by moving the preferred payment to the top of the list prior to the item being charged, and then change it back again.
Here's how to change the payment method order for your Apple account after Apple account balance (which will be charged first in all cases unless the item you are buying cannot be charged to account balance). For the first four steps, refer to the first image in this Support article Add a payment method to your Apple Account - Apple Support where it says to tap Edit to reorder or remove a payment method.
1. Tap on the Settings app.
2. Tap on the Apple ID section at the top of the page (your name).
3. Tap Payment & Shipping (on old versions this is iTunes & App Store).
4. Tap Edit.
5. Tap and hold on your preferred Card and drag it up or down the list.
6. Tap Done.