You may access your stored credit cards, and add a new one from both the mobile and web apps:
From the web app:
Use the menu in the top left hand side, click 'My profile' underneath your name. From there, click 'Payment Information' then 'Add new credit card', making sure that the 'save as default card' checkbox is checked.
From the mobile apps:
Use the menu to access “Settings” and you’ll be brought to your Account. Click on 'Payment Settings' then 'Add new credit card' to add this change information.
If you would like to remove your old card from your profile, please email email@example.com to have this removed.