I take you mean how to change the filter? If so open up the bonnet. Front right hand side of the engine compartment a flat plastic box construction. Te top unclips and hey presto cartridge type flat filter inside, lift out and replace with the filter element of your choice. Five minute job. Link below takes you to OEM site with a pic of the parts etc.
http://www.realoem.com/bmw/showparts.do?model=BT12&mospid=47795&btnr=13_0998&hg=13&fg=20