Disconnect the power from the light before opening the cover. Turn off the switch or flip the circuit breaker that controls the light's circuit at the main breaker box.
Remove the bulbs and place them to the side. If you are also replacing the bulbs, dispose of the old bulbs in accordance with local regulations. Florescent bulbs contain mercury, so take care when disposing of them.
Remove the metal panel that runs down the length of the light. Slide the panel edge from underneath the metal tabs that hold it in place. If the panel is secured with screws, remove these and place them to the side.
Cut the wires that are coming from the ballast. Try to cut the wires as close to the ballast as possible to ensure that you have adequate room to wire the new ballast in place.
Remove the ballast from the light fixture. Use a screwdriver or adjustable wrench depending on the type of screws that are used to secure the ballast.
Install the new ballast and secure it to the light fixture using the screws that you removed in the previous step. Be sure to tighten the screws securely.
Use wire strippers to remove ½ inch of insulation from the wires coming from the light fixture. Remove the insulation from the wires that are connected to the new ballast as well.
Match the wires from the new ballast to the wires from the light fixture. The wires are color-coded and should be connected to each other accordingly. If the wires on the light fixture are different colors than those on the new ballast, refer to the wiring diagram printed on the ballast for the correct connections.
Twist the bare wires together and secure the connection using electrical wire nuts. Tighten the wire nuts securely. Ensure that the connection is secure. Gently but firmly pull on the wires to ensure that they can not easily slip out of the wire nuts.
Replace the metal panel onto the light fixture and replace the bulbs. Reconnect the power to the light and turn the light on to ensure that it is working properly.