Lawn mower blades, including mulching blades, should be replaced every year. A sharp lawn mower blade is essential to having a healthy lawn. Dull lawn mower blades lead to shredding your grass instead of cutting it cleanly. A dull mulching blade is even more problematic because it won't mulch the grass effectively and may leave unsightly clumps over your lawn. Installing a mulching blade requires a few common tools.
Disconnect the spark plug and lay the lawn mower down on its side.
Slide the right size box wrench onto the bolt that holds the blade onto the lawn mower. Wedge a board between the blade and the mower housing or grab the blade with a gloved hand. This will prevent it from turning when you are using your wrench.
Turn the bolt counterclockwise and unscrew it completely. Remove it along with any washers that come off with it. Set these aside for re-installation later. Pull the old mulching blade off of the mower.
Put the new mulching blade into place. Look to see that it is positioned in the same way the old one was.
Screw the bolt back on and ensure the washers are in the same orientation that they were when they came off. Put the wrench on the bolt, grab the blade with your gloved hand and turn the wrench clockwise to tighten the bolt.
Set the mower back upright and reconnect the spark plug wire.