In order to update your account details or change your default billing method, log in to the client area and click on "hello <your name>" in the top right, and then select "account details".
On this page you can update your name, address, phone, default payment method, default billing contact and email preferences.
PLEASE NOTE: These account details are separate from your user/login details since any account can have multiple users.
Please see the article 'Account Owners, users & Contacts' for more details.
The specified Payment Method is what will be used for all your products and services.
The default setting is to have a payment method defined for each order. This setting allows you to have different payment methods for each product service.
If you prefer to just use the same payment method for EVERYTHING, you may change it here.
If you wish to add a new payment card, you can do this from the "payment methods" page.
Here is a video tutorial