Is there a way to "Edit" (update options etc.) an item already in the cart?
Not really. You can only update the quantity of items already in the cart. However, if you wanted to provide some form of inline edit product functionality on the checkout, it's possible to remove an item and add another item at the same time, so you could replace the item with a new one with whatever modifiers the customer selects.
To do this, simply create an add to cart form using the product tag, and pre-fill the form fields with the existing values from the cart.