Each time you cut your grass, your lawn mower blade becomes a little more dull. After a season of cutting, your blade may be very dull. A good way to check is to look at your grass blades. If they are cut cleanly, you have a nice, sharp blade. If they looked ragged and torn, your blade is dull. Dull blades can allow diseases and fungus to affect the grass through the ragged, torn edges. By installing new mower blades, you will help ensure a pristine, healthy lawn once again.
Disconnect the sparkplug wire from the sparkplug. This will prevent engine from starting when the blade is manipulated.
Lay the lawn mower on its side. Wedge a 2-by-4 board between the blade and the mower deck. This will keep the blade from turning when you remove the bolt holding the blade on.
Loosen up the bolt that holds the lawn mower blade to the engine shaft, using a box wrench. Remove the bolt and the blade should come off. Keep any washers that come off with the bolt because you'll probably have to reinstall these with the new blade.
Install the new blade in the same fashion the old one came off. Tighten the bolt down with your box wrench.
Set the lawn mower back upright. Reconnect the sparkplug wire. Your lawn mower is now ready to cut grass again.