Go to Billing->Make Payment, then the Credit Card Autopay dropdown section at the top. You can put your new credit card info in there. However, this is for billing every 21st of the month.
If you're owing money now, then the section below, Make a Payment is where you enter the new info to pay your bill. The top of the screen should show your balance.
If it says that you have $0 due, then contact support and ask them how much it is to renew your account.