Replacing the air filter on a Toro 22" Recycler Lawn Mower with SmartStow (model #20339 or 20339C) is very easy to do. It only takes about five minutes, and you just need the new air filter (part #491588S) and either a slotted screwdriver or a 7mm nut driver.
The air filter is located on the left side of the engine as you're standing in front of it. There is a black plastic cover protecting the air filter. One screw secures the cover; remove it with your screwdriver.
Now remove the cover by pulling it out from the top. The air filter will be exposed -- a flat-style air filter composed of pleated paper. Remove the filter, and if you have a rag or paper towel handy, wipe down the air filter housing to remove any dust or debris. You can also wipe off the air filter cover.
Install the new air filter, setting it into the housing so that the pleats face outward, away from the engine. It should fit snugly. Once the filter is in place, reattach the cover, making sure the screw hole lines up with the hole in the engine frame. Then put the screw back in and tighten it so the cover doesn't wiggle at all.