You can add a credit card to your account by paying for your invoice with a credit card payment method. After the payment is done, you will be able to see your card via the client area Account –> Credit Card:
You will be able to change your credit card details there, meaning change your card to another or remove it from your account.
Note. If a credit card is added to your account and automatic renewal is enabled for your service, the money from your card will be charged automatically the next day after the renewal invoice is generated.