After shipping your order to your customer, you can create a "return document" to keep track of items that are being returned by your customer and to return those items to stock (increasing your stock levels). Although most common for consignement sales, a return document can be created for any type of sale.
Creating a return
Navigate to the relavent sale, and choose "Return" from the "Create" drop-down menu at the top right. In the dialog, enter the quantities returned and click "Create".
You'll notice that the "return document" created is marked "NOT CHECKED IN". Use the "Check in" button at the right-hand side to add the units back into your current stock. If you wish to create a credit note for the returned items you can do that in the normal way.