Have you painted your walls with semi-gloss paint, only to discover that you hate the look because of the high sheen the paint produces? While it would have been better to consider the look before the area was painted, it is possible to reduce the sheen without repainting the entire surface.
Wash the area with warm soapy water to remove any grease, dirt, dust or grime from the area you are going to be working with. Wipe it dry with a clean cotton cloth. Allow it to completely dry overnight.
Sand the surface. Place either 400-grit (will produce a flat-like look) or 1,000-grit (will produce a satin-like finish) sandpaper on an electric sander according to the directions for the sander itself. Starting in an upper corner, gradually run the sander over the entire surface, being careful not to leave residual sanding marks. Change directions and movements often.
Wipe with a clean dry cloth once the entire surface has been lightly sanded. If the sheen is not to your liking, repeat Steps 2 and 3 once more. If after this, the look is still not agreeable to you, you will need to repaint. Since the surface has been sanded, you will not need to resand before painting over the semi-gloss.
Wash down the wall with warm water. Do this step no matter whether you have found the look you like or have decided to repaint the surface. This will remove any particles of sanded paint that may remain on the wall.