Update button for each line item in cart (drupal 7 commerce)
This is pretty simple to explain but building this will probably give you some headaches.


create a views handler (for the update button)
add the button in the cart view
done


Now the tricky part is building the form handler, you should take a look at commerce_line_item_handler_field_edit_delete which almost does what you need (only it deletes an item instead of saving it).

Since you only need to update the quantity you need to find out line item id and the new quantity. Load the line item and then update it. You don't need to update the order itself, commerce should handle everything when the order is updated.