fix replace ducted extractor fan in shower room?

Extractor ceiling fan in shower room seems to be running on "slow speed" ( seem to remember it used to be noisy in use and then quieten down to a slower speed for a while when switched off - now runs but same noise throughout)and have noticed more frequent build up of black mould specs recently I have hoovered dust from front vents but screw on cover is rusted on and unable to remove it is an old fan over 15years! Assume that it is ducted as there is a vent on the outside house wall. Would it be easy to replace? or is it a job for an expert?