There are two methods to add or modify the credit cards on your account.

1. In the cart checkout:

Once in the cart checkout, after choosing your shipping address, your shipping method, you'll arrive at the payment method section. Here, you may add a credit card (if you already have one saved, as shown below, click the drop-down option and chose to add a card). If you wish to modify the current one shown, you can click "Edit Card":

2. In your account:

Under your account, in the "credit card" section under "Payment", you can add a credit card as shown below:

Or, you can also modify the default one (the one which will appear first in your cart checkout), modify the information of your card, or remove one of the cards (as long as it's not the default one) as shown below: