Turn off the power to the ceiling light or fan at the home's main electrical panel.
Climb your stepladder up to the ceiling fixture. Remove the fixture by taking out the screws that hold it to the ceiling, using your screw gun. Untwist the wire connectors on each pair of joined wires. Pull the wires apart.
Go into the attic. Remove the bundle of wires that go into the electrical box on the floor (the ceiling of the bathroom). Loosen a clamp on the box with the screw gun to free the wires if necessary. Set the end of the wire bundle aside.
Remove the mounting screws that hold the electrical box to the floor support beams (joists) on both sides of the electrical box. Remove the box, leaving a hole on the floor. The bathroom will be visible below.
Place the new extractor fan unit over the hole, with the vent opening of the unit covering the hole. Draw the shape of the vent opening on the floor around the hole with your pencil. Move the unit out of the way. Cut out the new hole around the existing hole with your jigsaw.
Put the extractor fan back in place, with the vent opening dropping through the hole you cut. Secure the unit to the support beams on both sides, using the mounting brackets and screws that came with the unit.
Stretch the vent hose from the extractor fan kit to the exterior wall. Trace the shape of the hose on the wall with your pencil. Cut out the marked area to create an exhaust hole for the hose. Mount the hose to the wall with the provided screws.
Take the wire bundle you disconnected earlier and insert it into the electrical box on the extractor fan unit. Tighten any clamp that holds the wires in place in the box.
Connect the sets of wires. Match the bare end of the black wire from the bundle with the bare end of the black wire on the fan. Twist a wire connector tightly over the ends to secure the connection. Repeat the process for the white wires. Connect the copper ground wire from the bundle to the green grounding screw or green wire on the unit, depending on the design. Set the connected wires into the unit's electrical box and close the cover over them.
Go back down into the bathroom. Set the fan's vent cover over the new opening you cut in the ceiling, installing it as directed -- some models require screws, others hook on. Turn the power back on at the electrical panel.