To edit a card in list view click the card #. On touch enabled devices you can also swipe left or right on the card to start the edit process.
To edit a card in card view click the edit card icon.
When editing a single card you have the same options as you do when creating or editing a set.
When done editing click Save to save changes or cancel to discard them.
That's it. Now you don't have to edit a whole notecard set just to edit one or two cards.