How do I update my payment information?
Visit the Payments page, linked in the top navigation dropdown.
If you've already entered a card and want to change the info, press the Use a Different Payment button and enter the new card info in the popup that appears.
If you need to update your payment information because it failed, but it was a temporary failure and the card should work now, then you can enter the same card again. You'll get a chance to accept or reject the charges, and confirming your new card (and paying the outstanding charges) will clear the non-payment flag. The top of the modal will display the amount you're going to pay once you link your new card, and there are links on each charge to allow you to mark them as non-legit separately.
If you've entered multiple payment methods (e.g. you also have a PayPal account linked, from when it was possible to add PayPal), you can choose one to use as the default for all new charges by checking the Default button below it.
Keywords: updating your payment method, fixing an expired credit card, payment method failures, the unfortunately named deadbeat flag