Rotary mowers use a gasoline powered engine to spin a lawnmower blade that cuts your lawn. The lawnmower blade cuts the grass by slicing through it at a very fast rate of speed. As the blade begins to dull, instead of cutting through the grass cleanly, the lawnmower blade will begin to rip and tear at the grass. This jagged edge will allow fungus and other diseases entry into the lawn. To prevent this it is essential to replace your lawnmower blade each year.
Remove the gas from the gas tank by letting the lawnmower run until it's out of gas. Push the lawn mower to a level, well-lit area.
Remove the spark plug wire from the spark plug, and turn the lawnmower on its side.
Wedge a wood block in between the lawn mower blade and the edge of the mower deck to stop the blade from turning when you remove it.
Look for the bolt that holds the lawn mower blade to the engine shaft of the lawn mower. Place an adjustable wrench on this bolt, and turn it counterclockwise to loosen it up. Unscrew completely and remove. Set aside any washers that came off with the bolt; you'll put those back on with the new blade.
Remove the old lawnmower blade, and put the new one on. Put the washers and bolt back into place.
Tighten the bolt with your adjustable wrench. Remove the wooden block, and set the lawnmower back upright.
Reconnect the spark plug wire to the spark plug, fill the lawnmower with gasoline and start it up.