Turn off the power to the area in which you will be working.
Unscrew the switch cover plate and remove it.
Pull the old switch out carefully to expose the wires.
Unscrew the wire nuts to remove the wires.
Take the old switch to the store to get a control switch that will fit perfectly. It should have the same number and type of wires. You will also need a new cover to fit the switch.
Reconnect the wires from the wall to the control switch.
Once the wires are firmly attached, mount the switch back in the wall and replace the new cover.
Screw the cover in place and turn the power back on.