First, make sure you are signed in to the Customer Portal. Choose "Payment Methods" from the menu on the left. Here you will see your current Payment Method and have the option add a new payment method.
To add a new card, click "Add Payment Method". Enter your credit or debit card information. You also have the option to "Update the Payment Method used for all of my active subscriptions." Be sure to click the yellow button "Add Payment Method" to save the changes on your account.