To replace a bathroom vent fan, remove the old vent fan motor and housing, adjust the hole in the ceiling and install the new vent fan. If your current vent fan is annoyingly loud, the newer models operate much more quietly and use less energy than previous generations.
Put an old quilt or a plastic tarp down on the floor to shield it from damage if something falls during installation. Turn off the circuit switch that powers the bathroom. Take off the cover of the old fan. This usually involves pulling down it, as pressure clips hold it in place. Unhook the wiring, and take the motor out of the housing. If you are putting in a completely new housing, take out the screws holding the housing to the joints.
Measure around the new housing to see if the ceiling hole needs adjustment. Use a jigsaw to enlarge the hole in the ceiling. Connect the new housing to the joists. Connect the wiring to the housing, and connect the duct to the housing. After installing the motor and finishing the wiring according to the directions in the manual, put the new cover or trim piece in place. Turn the power back on to the bathroom, and test the fan for proper performance.