How do I replace the cabin air filter and where is it?

 

I believe it is above the soundproof panel which is the panel that would be above your knees on the passenger side near the pedals. Slacken off the 2 screws.
Remove the soundproofing panel by pulling it straight down.
Replace the soundproofing panel

Remove the screws for the cover
Pull the air cleaner (ACL) downwards / away.

Replace filter, install it back in and put the panel back on. Don't tighten down the bolts too tight.