Disconnect power to the bathroom, then connect an extension cord to an outlet on another circuit and run it into the bathroom. Plug the utility light or lamp into the extension cord so that you have light in which to work.
Remove the light bulbs from the original light bar.
Remove the screws from the top, bottom or sides of the light bar, depending on the model. These screws secure the light bar cover in place, which you can take off after taking out the screws.
Identify the wires connected to your existing light fixture. The hot wire will be either black or red, the neutral wire is the white wire and the ground wire will be either bare or green.
Disconnect the wires connected to your old light fixture.
Remove the fixture from the wall by unscrewing any mounting hardware connecting the fixture to the wall. Often, the mount hardware will be located around the hole in which the wires are inserted.
Refer to the instructions of your new light fixture for the proper mounting procedure. Often, the mounting bracket will screw directly onto the junction box in the wall.
Connect the wires from your new light fixture to the wires on the wall. If necessary, use the wire strippers to strip the wire casing. Twist the exposed ends of the matching cables, then insert the connection completely into a wire nut. Twist the wire nut to further secure the connection.
Tuck the wires into the wall or junction box, then complete the installation by mounting the fixture according to the manufacturer's instructions.
Restore power to the bathroom once the fixture has been installed to complete the installation.