Retrieving completed order on the KDS
Tap on the calendar icon on the top right side of the page and it will come out with a list of the all completed orders
Once you have found the order you want to retrieve, select MARK AS UNDONE and there will be a prompt on the screen to confirm that you would like to retrieve it
Select YES and then the order will be brought back to the queue
Here's a step-by-step video guide: