The Toro 22" Low Wheel Recycler Self-Propelled Mower (model #20350) uses a flat, pleated air filter similar to many other mower models. Changing the air filter is very easy to do.
The air filter (part# 491588s) sits behind a flat black plastic cover, on the left side of the engine as you're looking at it from the front. To remove this cover, unscrew the hex head bolt located in the top middle part of the cover with a flathead screwdriver. (If you have a socket wrench that will fit, that will work also.)
With the bolt out, pull the cover out and down, exposing the air filter. The air filter will come right out, and you can set the new air filter in its place (with the pleats facing outward). Then put the cover back on and secure it with the bolt, and you're done.