You can make changes to your payment method from your "Settings" tab in the "Account" dropdown menu, then navigate to Payment Methods in the left navigation bar. The only forms of payment accepted at this time are credit cards.
You can set your default card by clicking on "set default". Please note that your default payment method will be the card charged at the time of your invoice due date.
If you would like to add a new card, click "Add Payment Method" and you will be prompted to fill out the card number, expiration date, CVC, and billing zip code.