To update your credit card information as a single account user please go to settings in the top right corner of your screen.
After you are in settings, you will see the page below. Click on "Billing Info" on the bottom left hand corner of the screen.
Once you click on billing info, the page below will appear. Click on the "change" button to give you the option to update your information.
Once you hit "change" the screen below will appear and allow you to enter new billing information.