The interior cabin air filter is located in back and to the right of your gas pedal. You need to take the trim plastic piece off next to your gas pedal. It just pulls straight off. Then locate the "L" shaped plastic filter cover in back of the gas pedal. You will need to put a flat head screw driver under the bottom to pop it off. Once off, you will see the white filter. Make a hook out of a paper clip and pull the filter out. Vacuum with a thin nozzle the slot where the filter came out since it will be real dirty. Place the new filter in observing the air flow arrow air flow in from outside so arrow points in. Also buy your filter at the dealer. I've tried several online sources and one auto parts store and all sold me the wrong size filter. For some reason none of these after-market sources have it right. They come with instructions showing glove box install which is wrong and I believe Altima instructions.
Further, some say you need to remove the gas pedal this is not true. You can work the filter around it.
Nissan if your listening "CUT ME A BREAK"

you need to redesign this in future models it's way more difficult then it should be! WHY did you put it in back of the gas pedal and not in the Glove Box like other Nissan Models ???
If your more visual you can view my video on how to change the filter on YouTube. Here's the link :
YouTube - MVI 2417
Hope this helps. Do it yourself whenever you can!