Select "Billing" from the top menu, then select "Manage Credit Card" from the drop down sub-menu.
If you have any oustanding Invoices, you might need to change your payment method to the new credit card.
Click "Billing" from the top menu, then select "My Invoices" from the drop down sub-menu.
Click on the unpaid invlice to open it in a new window.
On the invoice, under your information, there will be a dropdown list called "Payment Method". If needed, change this to Credit Card.
On the top right of the invoice click the "Pay Now" button.