After adding a slab edge to a floor, you can change the size or orientation of the slab edge.
You can resize slabs. When you select the slab, a drag control appears at either end of it. See Controls and Shape Handles.
You can flip slabs by using the Flip controls that appear in 3D, or, if you are in a 2D view (such as section), you can right-click on the slab and select Flip About Horizontal Axis or Flip About Vertical Axis. See Controls and Shape Handles.