Turn off the power at the circuit breaker panel. Test the wires to make sure there is no current flowing through them.
Remove the old hood first. Near the light fixture you'll see a cover that protects the electrical wiring. Disconnect the wiring by removing the wire nuts and separating the wires. Have your assistant hold the hood in place while you loosen the screws. Slide the hood off the screws and remove the screws.
Check to make sure you have at least 12 inches of wiring coming out of the wall to connect your new hood. If your new range hood will vent to the outside, proceed to Step 4. If it is a vent-free hood, skip to Step 5.
Locate the ductwork for the fan; it will either travel upward through the cabinet or back into the wall. Open the venting hole on the range hood that corresponds to the location of the ductwork.
Locate the metal cover next to the light on the new hood. Like the old hood, this is where the wiring is housed. Remove the cover.
Have your assistant hold the new hood in place while you mark the location of the screw holes on the underside of the cabinet. Put the hood down, and drive the screws about halfway into the places you marked. Slide the hood over the screws, and make sure the opening in the hood aligns with the ductwork (if you're venting to the outside).
Guide the wiring from the wall through the hole in the back of the hood. Connect the two black wires from the hood to the black wire from the wall using wiring nuts. Connect the two white wires from the hood to the white wire from the wall using wiring nuts. If there is a green wire from the wall, attach it to the green grounding screw. Replace the metal cover.
Tighten the mounting screws to secure the hood.
Turn on the circuit and test the light and fan. If you're venting to the outside, check the vent to make sure the air is flowing freely.