The Reverse Plate Direction tool lets you reverse the direction of plates, effectively swapping their front and back faces. It also results in the plate’s local x and z axes having their directions reversed.
After selecting the selected plates, right-click and select "Plate Tools" => "Reverse Plate Direction" from the popup menu that appears. If you tick the "Adjust the direction of loads so that they are unaffected by the reversal" option then any plate loads will be adjusted so that they remain in the same general direction as before the plates were reversed.
Note that the order of the nodes around a plate are changed after the plate has been reversed.