You can easily update your payment method within your account via the Billing area.
Navigate to: Billing -> Billing & Payments
Select the option: Payment Methods
Fig 1 - Billing & Payment options
The Payment Methods screen will detail all the cards you have entered to pay for your account, to add another card to for either a Secondary backup or to replace the existing card, press [Add New]
Fig 2 - Add a new card
Enter the card details and press [Add]
Fig 3 - Adding a new card
Once the new card details are saved you can set it to be either the Primary or Secondary card for payment.
Fig 4 - Setting priority to cards
If you wish to remove a card go to the Payment Methods screen, select the relevant card and press 'Remove Payment Option', once confirmed the card will be removed from the account.
Fig 5 - Removing a card
Please Note - Primary cards cannot be removed, if you are changing to a new card, add the new card and set it as 'Primary' once completed the old card can be removed.