How to change the material of the exterior wall?

Switch to 3D view, hover the "View" button on the top toolbar, and then untick the "Hide Wall" option to display the exterior wall (if you tick this option, then the frontal walls will be hidden automatically in order for you to better view the interiors). Select the exterior wall, you can replace the material of the exterior wall, or you can click "The Floor" to customize the tile of it like drawing shapes, dividing the area, and applying different materials to it.