Once the Item is added into order, Its attributes like quantity and discounts can also be modified. Follow below steps to modify item details after being added to the order
From here, You can increase/decrease quantity of item or you can add discounts on this item.
If you wish to remove selected Item, click on “Delete” button from the header to remove this item.
Once all the changes are done click on “Save” button in the header to finalize your changes.
Your modified details will be visible on the item.