The Lawn-Boy 20" Push Mower (models 10640, 10641, 10642, 10645, 10646, and 10647) uses a flat, pleated paper air filter to keep contaminants from entering the engine. This filter can easily be replaced with just a flathead screwdriver.
The filter cover, a rectangular piece of black plastic, is located on the left side of the engine, as you are looking at it from the front of the mower. Remove the hex head screw located in the top middle of the cover, then remove the cover itself. The air filter will be exposed -- remove the old filter and install the new filter in the same orientation, with the pleats facing outward. (If you have a rag or paper towel, you can wipe down the air filter housing to get rid of any dust or grass that got caught there.) Then reattach the cover and secure it with the screw.