Assuming you want to remove an outlet cover so that you can access the screws holding the outlet in place:
To pop off an outlet cover, first locate the small tab at the top or bottom of the plate. Gently insert a flathead screwdriver or other thin object behind the tab and pry it away from the wall. The cover should come loose and you'll be able to remove it without damaging it. If there is no tab, look for two small holes on either side of the plate. Insert a paperclip or other thin wire into one of these holes and push until you feel resistance. Then do the same with the other hole. The cover should now come loose and you can pull it away from the wall.