Remove mounting screws on switch cover plate and take off plate. With cover plate removed, you'll see two screws holding switch in switch box. If necessary, remove screws, and carefully pull switch out of box as far as attached wires allow. If there are two screws with wires attached, switch is a simple ON/OFF (single-pole) type. If there are three screws with wires attached, you're working with a more complicated type called a three-way switch. Replacement switch must be the same type as old one, either single-pole or three-way. Three-way switches allow you to turn light on and off from two different locations, such as at top and bottom of stairway. Look carefully at three terminal screws; you'll see that two are one color while the third is a different color. Do not disconnect any wires until you compare old switch with replacement switch so you know which wire goes to which terminal screw.