When you add a payment method for the first time, it is saved in your account and listed as your default. This means that in case of a future purchase it will be automatically used to enable you to perform the transaction without you having to re-enter the respective information. However, it is possible to add a new card or change your default payment method easily. In order to view your payment method list, click on the Billing tab of the My Account section, and choose the Payment Methods pane.
On the right-hand side, you will notice that the field to add a new card is always available. Simply enter your card details and click on the Add Payment Method button. The newly added card will be automatically selected as your default payment method.
When you have more that one payment methods, you will be able to view a list of them in the Payment Methods pane. Next to each card, there is a button to Make Default and another to remove a payment method.