The Restore button in the "Shipped" orders screen will move orders back to the Awaiting Shipment view on the orders tab.
To Restore an order, you'll need go to your Orders tab, then into the Shipped view. Select the order(s) and then click the Restore button at the top of the screen.
This option can be helpful if you've accidentally marked an order as shipped or need to indicate to your warehouse team that they need to re-ship that order.
Note: If the order on your selling channel has a Shipped or Completed status, a store refresh could update the status of the order in ShipStation back to Shipped. So using this method is best when you plan to immediately create a new label for the order to be re-shipped.