You will receive an email approximately ten days before the end of the month warning you that the card you've entered is about to expire, and encouraging you to enter payment details. You can update this information at any time by signing in to the developer center, selecting the "Account" link, and
then selecting "Credit Card Details."
If you have not updated your Credit Card information by the time we attempt to charge your card, you will receive an email about the failed charge and again be reminded to update your payment details.
(See "What happens if my credit card bounces?")