Go to the Sales → Orders → Edit Order
The details of an existing order can be changed or updated by the store owner.
There are various reasons why you might need to edit an order after it has been placed. The Customer can contact you with a shipping address correction request or if you do not have enough stock for an order or receive a request to change delivery method.
To add products to the order, search with Add Product field. A list of products will appear. Select the product you wish to add then input quantity and options if available
Save button will save all your manual changes otherwise Save & Recalculate button will ignore your manual values and calculate all total cost based on total rules (the tax total based on the customers shipping address).
Every time you edit anything in the order a manual review is required! Order edit within your admin will not automatically send the changes to your payment gateway!