Go to FooSales.com > My Account > Plans to view the details for your plan.
Click the Change Payment button then select a different payment method and enter your new payment details (payment methods vary based on your region).
Please remember to select the Update the payment method used for all my current Plans checkbox to update the payment method for all your current plans, otherwise you will need to change each of them individually.
Click the Change payment method button for the changes to take affect.