That's not supported as that's not a typical use case... the only 'edit' option you have is to remove the item and add it back in. There's no interface for even editing this - all you have is a product form which can optionally contain info that you type into per templating, but that has nothing to do with the item that was then added to the cart. They are two completely separate entities.
You could potentially create something of your own to edit a line item. Probably an AJAX pop over form that would physically edit the line item data...
+++ Rick ---