1) From your Portfolio account:
Once logged in into your Portfolio account, click go to your "Settings" located on the toolbar on the left, 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.