Shut off the breaker at the main electrical panel powering the outlet or switch.
Locate the small standard screws on the plate. Look for a single screw between the two outlets, or the screws at the top and bottom on a conventional switch. Multiple switches may have four or six screws, depending on the number of switches.
Rotate each screw counterclockwise until the screw comes out. Carefully pop the plate off the wall. You can gently score around the edge of the plate if it's stuck to the wall with paint.