You can update your default credit card from your Account Settings. On the homepage, hover over your name at the top right corner of the screen and choose "Account Settings" when the menu appears. Next, choose "Credit Cards" at the top of the following page.
If you're adding a new credit card, you can check the "Make Default" box that appears directly below your billing information.
Any credit cards already saved to the account will appear above the new billing information section - the option to make any card the default card will appear to the right of the card information.