Replace a three-way light switch by unplugging the lamp, removing the shade, harp and light bulb, pulling the outer housing off the socket and loosening the wires from the screws. Loosen the set screw, unscrew the socket from the threaded rod, and pull the socket off the wires. Reverse the process to install the new socket and switch. On most lamps, replacing the three-way switch also requires replacing the lamp socket.
When connecting the wires to the new switch, observe the color coding. Most lamps have a dark wire and a light one. The dark wire connects to the brass screw on the switch and the light one connects to the silver screw, ensuring the correct polarity of the wiring.
Three-way lamp switches have a four-position rotary switch. These lamps require special bulbs that have two filaments. The settings on the switch direct the power to the lower-wattage filament, the higher-wattage filament or to both filaments, allowing you to choose the amount of light the lamp provides. If a three-way lamp does not illuminate on all the settings, check to make sure that the bulb in use is a two-filament bulb and not a standard, single-filament bulb. Since replacing the switch requires replacing the socket, it is easy to convert a standard lamp to a three-way device by choosing a three-way switch and socket.