How can I change the payment method for my AppleCare subscription?

Try going to Settings > General > AppleCare & Warranty. There might be an option to "finish AppleCare setup" which can help connect your AppleCare subscription to your Apple account. This should allow the fees to be charged to your Apple account payment method instead. If it doesn’t work, your best bet is to call Apple Support directly; they can guide you through the process of changing your payment method.

If I understand correctly, the payment will automatically be taken from the card at the top of your list. I had a similar issue in the past. I ended up changing the order of my cards and since then I haven't been charged for AppleCare on the wrong card anymore. Just keep in mind, you have to make sure your Best Buy card is the first one listed.

Go to Settings > your name > Subscriptions > AppleCare+ to see if there's an option to change the payment method there. It might just let you update it directly from the subscriptions area.

Did you get your AppleCare through Best Buy? You can tell by the billing statement; if you see "AppleCare... Minneapolis, MN", it means Best Buy is handling the billing. In that case, you need to log into your Best Buy account and update your payment method there instead of Apple. If the charge appears as "apple.com/bill" or "Apple Services", then Apple is charging you directly.