Right click on it and select reverse face.
To reverse what exactly pleased?
You have two floors. On the upper one you just need to not hide the slabs. To show the upper slab on the lower floor plan I would create a separate view with the upper slab only, and stack it on the ground floor plan in LayOut, as a wireframe view, with a dashed/dashdot line type
What about the down slab 1 and the wall that join the exterior rounded pillars?