Lay down in the passengers footwell. Under the glovebox is (or should be) a carpet like cover thing, remove that and then look for a L shaped cover under the HVAC unit towards the centre of the car. Held in by slightly wierd 5.5mm hex head screws.
Have a look for a guide with pics on the web it'll make it much easier as it's quite well hidden.
I did my 147 earlier this year. Problem is, I could only get one of the 2 parts out. No way can I get the 2nd filter out as it's too far into the housing. The bit I could replace was the 1st of the filters, so the 2nd one which I can't get out should be clean enough.