Your mulching lawn mower works hard to mulch the grass up into small particles so that it can effectively decompose back into the soil. As the blade repeatedly cuts the grass, it slowly dulls over time. Also, if your lawn mower picks up any twigs and small stones, these will hasten the dulling of your mulching blade. At least once a season, it is recommended to sharpen mulcher blades---more often if you run into twigs or small stones regularly.
Turn the lawn mower off and take it to a well-lit and level surface. Disconnect the spark plug wire from the spark plug.
Lay the lawn mower on its side and remove the mulching blade from the lawn mower. It is easily removed by loosening the nut that holds it to the engine shaft. Use an adjustable wrench for this.
Use a wire brush or steel wool and remove any rust from the lawn mower blade.
Put the lawn mower blade in a table vice to keep it stable. Use a metal file or angle grinder to sharpen the mulching blade. Follow the natural angle of the blade and sharpen until there is a nice, smooth and sharp edge on the blade.
Remove the blade from the table vice and check its balance. Support the blade in the middle and notice if it tips either direction. If it does, remove a little more material from the blade on the side that tips down.
Reinstall the blade back on the lawn mower by putting the bolt in and tightening it back up with your adjustable wrench. Stand the lawn mower back upright and reattach the spark plug wire.