It's not easy unless you're 4'2 with tiny hands!!!

Disconnect the battery. Pull the passenger seat back as far as it will go then take out the glovebox so you can see better (three screws at the bottom). An inspection light is useful. In the passenger footwell, there is a felt cover at the bottom fastened onto the plastic fascia (three more screws). Remove this. Then remove the short plastic ducting that runs from the central block to the footwell. You can pull it out by hand. The fan is fastened into a plastic casing with three more screws at the bottom of this casing and fastened vertically. You'll need to more or less put your head in the footwell to see and remove the screws with a stubby phillips screwdriver. Remove the connection block also located at the bottom of the motor, then the whole motor and fan drops down into the footwell. It all goes back in reverse order. If there's any rusting, its likely that the seal between the windscreen and the pollen filter has gone as well so you may need to replace that. As I say, the difficulty is getting access to the thing. For me, putting it back was much easier than getting it out. If you search the other forums regarding this subject, someone has posted a useful pic. Hope this helps. Good luck! Chris.