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
So there you will be able to change your credit card details, meaning to change your card to another, or remove your credit card 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 be generated.