This brico tutorial explains how to change the air cabin filter on a Ford Focus I. This brico is valid for all the Ford Focus from the first generation.
It's an easy maintenance operation which must be done each 15.000 Kilometers or at least yearly. Fit the dust pollen filter in the Ford Focus won't take more than 20 minutes and no special tool is needed to replace it, just with a screwdriver this maintenance operation could be done.
The air cabin filter could be shopped in nearly all the auto part shops, of course is also available in the auto spare shops online.