A three-way switch is a handy convenience to control a light from two locations, such as at the top and bottom of a staircase. If the words On and Off aren't embossed on the switch and it's one of two switches that control a single light or receptacle, you have a three-way switch. Seems like it should be called a two-way switch, right? The name refers to the fact that these switches have three terminal screws.
To replace a three-way switch, follow these steps:
Turn off the power to the switch at the circuit or fuse panel.
Unscrew and remove the switch plate; then use a voltage tester to make sure that the circuit is dead.
Unscrew the switch from the electrical box and pull it out with the wires still attached.
A three-way switch has at least three wires, and possibly four, depending on whether it has a ground wire. Two wires attach to brass screw terminals, which are usually at the top of the switch, and an additional wire attaches to a dark-colored (not green) screw terminal, which is usually at the bottom of the switch. Mark this third wire with a piece of tape and mark the wire on the same side of the switch directly above it with a piece of different-colored tape.
The new switch may have the electrical screw connectors in slightly different locations than the switch you're replacing. Most switches have a pair of terminals on opposite sides of the switch top and a single terminal at the bottom.
Remove the wires from the switch.
Attach the tagged wires to the corresponding terminals of the new switch.
Alternatively, you may choose to transfer one wire at a time from the old switch to the new switch.
If the existing switch has a green ground wire, attach the wire to the green screw terminal on the new switch or to the electrical box.
Push the new, wired switch back into the electrical box and screw it in place.
Screw on the switch plate and turn on the power.