Replacing the air filter on a Troy-Bilt 21" Self-Propelled Mower (including models TB210, TB230, and TB280Es) is an easy task and should only take a couple of minutes.
As you're standing in front of the lawn mower, the air filter is located on the left side of the engine (the opposite side as the muffler). It's hidden behind a rectangular black plastic cover. Remove this cover by taking out the screw in the top of the cover with a flathead screwdriver. With the screw out, you can pull the cover away, exposing the air filter.
Remove the old air filter, and install the new filter so it faces the same way, with the pleats facing out (away from the engine). Then put the cover back on and secure it with the screw.