To change your billing info simply login to your account with the email address and password you signed up with.
In the Credit Card Box click the Edit button and then click in the Number box and start typing to overwrite the current number. It will looked grayed out but it will overwrite what is in there (no need to delete first!).
Enter your new information, click save, and you're done!
Kindly note that if you pay through PayPal you'll have to login to your PayPal account to update your payment info.