Very odd gas furnace installation in apartment... How do I change the filter?
Can't tell for sure, but it appears that the return air intake is on the "back" of the furnace.



You'll have to measure the opening, and then buy a return air grill that can accommodate a filter.



You may have to install a bit of ducting, or seal the grill to the furnace using some foil tape or mastic. You'll want the return air to all be pulled through the filter, so you want the grill sealed to the furnace cabinet.

Once it's installed, you'll just have to pop open the grill to replace the filter.