Closed invoices cannot be changed. An invoice can only be cancelled completely. With these two methods, you can still post the cancellation correctly:
Sell the product with a negative quantity
Find the product being returned
Add it to a new invoice
Click on the product on the right side of the POS
Click on the minus sign to put the quantity in the negative range
Sell the product or add other products to the invoice
Cancel the entire invoice
Find the entire original invoice in the admin area or on your POS
Cancel the found invoice completely
Important: This method postpones the entire turnover to today and is especially suitable if the goods have been completely cancelled.