You have 3 options to return the products:

Option 1: You can delete the order shipment of that order (This will create the stock movement and add the qty to the warehouse). Then the order will be moved to the Order preparation screen again, you can create the shipment label and send the parcel.

Option 2: You can duplicate the Order from the CMS and re-import to your MyFullfillment and process the order.

Option 3: Create a RMA and select the "Exchange" method. Add the same product from the warehouse and ship normally with zero euros.