If you are logged in to your Dropbox Fax account right now, you can visit this page to update the card on file.
You can change your credit card information by signing in and finding and hovering over your email address in the upper right corner. Then click Settings from the drop down menu that appears:Click on the Billing tab at the top and then on the green Update button to change your credit card information.
Note: Only team admins can change the billing information for a team. If you aren't a team admin, you will not see this option to update your team's billing information in your account.