1) From your Portfolio account:
Once logged in into your Portfolio account, click on Edit your Settings located on the Toolbar, then head over to Your Account -> Manage Your Account.
This will take you to https://accounts.adobe.com where you will be able to edit or add new credit card information under Plans and Products -> Manage Plan / Add Payment Information.
2) On accounts.adobe.com:
You can also directly log into https://accounts.adobe.com with your Adobe ID, head over to Plans and Products -> Manage Plan / Add Payment Information and edit or add your credit card information from there.