A sharp lawn mower makes a cleaner cut as it passes over your grass. Cleaner cuts result in thicker, healthier lawns. Although lawn mower blades can be sharpened over and over, there comes a time when you may need to replace your mower blade either because of age or because it has struck a rock or other large, hard object. In some cases, your mower may be outfitted with a straight blade and you may want to install a mulching blade.
Disconnect the spark plug wire and turn your mower on its side. Make sure the carburetor is facing up to prevent gas spills. The carburetor is under the air filter.
Slip a block of wood between the blade and the side of the mower deck to keep the blade from spinning.
Loosen the bolt or bolts holding the old blade in place with a correctly sized wrench. Some older mowers have a single bolt in the center. Newer mowers may have two bolts.
Make a note of the direction the cutting edge is facing. Remove the blade and set it aside.
Place the new blade in place on the mower's mount. Make sure the cutting edge faces the same direction as the old blade.
Insert the bolt or bolts and tighten them down. If your blade came with new bolts, use the new bolts. If necessary, use the wood block to keep the blade from spinning.