Creating a return is as simple as setting the quantity to negative. For example, if a customer is returning 2 tshirts, enter the quantity as -2.
If you disable selling out of stock items as the setting below, you have to mark the invoice as a return.
Go to Store > Click Tools > Return Items
In the Return modal, input the invoice number that related to the item to be returned and the reason. Click Save.
You'll be redirected back to the store. Add the item to be returned and change the quantity to negative. For example, if the customer is returning 1 item, change the quantity to -1.
A return invoice will be created (you can view this in your Office > Invoices) and the quantity of the returned item should be updated in your Stockroom.