Maintaining a sharp blade is the only way to ensure a clean cut on turf grass. Cutting cleanly preserves moisture and minimizes exposure to disease and insect damage facilitated by jagged or shredded blade ends. Reel mower blades are self-sharpening, but rotary blades must be replaced.
Spread out a drop cloth. Assemble your tools in an area where you can reach them, and put on some gloves. Wipe or whisk grass and dirt off the mower housing. Detach the wire that connects the spark plug to the ignition to prevent an accidental startup.
Tip the mower on its side with the air intake and carburetor--the small box to one side of the engine--on the up side. Wedge a scrap of wood between one end of the blade and the skirt of the housing to hold the blade steady while you remove its mounting bolts.
Hold the loose end of the blade as you remove the mounting bolts and any washers. Some blades attach to the drive shaft with one bolt, and some have a mounting clamp with two bolts. Mulching blades may consist of two separate blades. Check with your owner's manual or just note the way any lock washers, clamps or other mounting hardware is arranged as you take it apart.
Pull the blade straight off the mount. Lay out the bolts, washers and any clamps in the order they should be reassembled to avoid confusion.
Clean out the underside of the mower body with a broom. Scrape off compacted grass with a putty knife before replacing the blade.
Place the sharpened or new blade on the drive shaft or clamp. Be sure the new blade sits on its mount the same way as the old one.
Replace the bolt and any washers in the reverse order of their removal. Hand-tighten the bolts and then tighten them firmly with a wrench.