To update your billing information, you would login to your client area:
Once logged in, click the Billing dropdown > Payment Methods
You can manage your credit cards from that section. Once this has been updated, you may want to make sure that your Billing Contact is setup appropriately by clicking Edit Account Details and doublecheck that the Default Billing Contact is correct.
If you cannot remember the password for your client area, there is a "Forgot Password?" option under the login form, which will send you an email with a password reset link.