You can update your payment info at any time.
Step 1 - From the drop-down (top-right photo), click Account Settings.
Step 2 - On the left-hand side you'll see a line called "Payment Methods." Click this.
Step 3 - From here you can add a new credit card or PayPal account. You can also transfer your pledges to a new or existing payment method by clicking the "transfer pledges" link.
NOTE: If you delete your payment method, monthly pledges will be deleted unless you transfer those pledges to an existing or new payment method. This will not delete pending payments from paid posts that were made when you were pledging.
If you are having problems updating your payment info Check this out.