An extractor fan is typically used to vent moisture from a bathroom to prevent mould from forming. To adjust the time when the extractor fan turns on before shutting off automatically, you must change the setting on the control panel inside the fan's casing. You will need household tools, but no special equipment, to perform this adjustment.
Turn on the light in the bathroom where the extractor fan is located. Flip the extractor fan's switch to turn it on as well.
Trip the appropriate circuit breaker in the fuse box to discontinue power to the bathroom. Tape the fuse box closed with strips of duct tape or masking tape. Lock the door leading to the fuse box, if you have one, so no one can access the fuse box while you are adjusting the time on the extractor fan.
Check to verify that the light in the bathroom is now off and that the extractor fan is not running. Flip both switches a few times to confirm the lack of electricity in the bathroom. Plug an electrical device --- for example, a hair dryer --- into a AC outlet in the bathroom. Turn on the electrical device to reconfirm the lack of electricity in the bathroom. Unplug the electrical device, and put it away.
Put on a pair of insulating gloves as a safety precaution. Place a rubber mat on the floor beneath your feet as yet another safety precaution.
Unscrew the faceplate of the control panel connected to the extractor fan using a Phillips screwdriver. Pull the faceplate off the control panel, and place it and the screws on the bathroom's countertop.
Locate the green circuit board inside the control panel, typically at the right side. Locate the black plastic plug with the single screw you'll typically find to the left of the circuit board, although the positioning of the two might be reversed in some cases.
Insert the tip of a plastic slotted jeweller's screwdriver into the screw. Rotate the screw two full revolutions clockwise to increase the amount of time the fan will stay on before shutting itself off.
Place the faceplate back on the control panel, and reattach the screws. Return to the fuse box, and remove any tape holding it closed. Flip the circuit breaker you turned off earlier back to its "On" position to restore electricity to the bathroom.
You may need to stand on a wooden chair to reach the extractor fan.
Inform any family members or other people in the house that you are working on the extractor fan and that they should stay out of the bathroom and not go anywhere near the fuse box.