Mower blades can be sharpened a few times, but after the third of fourth sharpening they need to be replaced.
If you need to replace a lawn mower blade, you can get the blade to match your lawn mower at http://www.mrmowerparts.com/
Step 1: Unplug the spark plug to prevent any unsafe repair conditions. A precaution perhaps, but best to heed.
Step 2: Drain gas with syphon hose; self syphon pumps are sold at any hardware or automotive parts store. This prevents gas spillage onto the mower body.
Step 3: 3Check your manufacturers instructions to find out which side it's safe to lift up your mower deck because oil can spill into exhausts and/or carburetors etc on some models if tilted the wrong way. Lift mower deck and prop up as the blade requires two hands to remove.
Step 4: Use a piece of wood (IE a 2x4) to prevent blade from moving while removing blade mounting bolt.
Step 5: Using a socket wrench set, determine the size of the blade mounting bolt for removal. Unbolt while using other hand to hold blade from turning. Be careful not to lose any washers or mounting hardware that hold the blade into place.Remove brace used to keep blade still during removal.
Step 6: Observe the position of the blade during removal. You will have to mount the new one exactly as the old in order for it to cut the grass rather than merely pushing the grass downwards.
Step 7: Mount the new blade which should already be pre-balanced from manufacturer. Most blades are either specific or universal fit. Be sure you have the same length before mounting the new one as the blade must have clearance from the deck itself.Wedge 2x4 or other into blade prior to remounting for safety as blade can cut you when new.Tighten, but do not over tighten in reverse order from the removal.
Step 8: If your remounting a used or resharpened blade be sure to balance using a balance tool or a screwdriver placed into bolt hole.Must remain level or your going to have vibration issues with your mower.
Step 9: Be sure the blade is mounted correctly and has no wobble or play to it. Remove any jacks or props used to hold mower in place. Wait about 30-60 minutes for oil to return to motor to prevent issues or motor damage. #Check the oil before use to ensure it is within the proper limits.
Step 10: Refill the gas tank and do a pre-check. Check the air filter to be sure that oil did not saturate the foam filter if you have one. Attach spark plug wire.Better to recheck than have an issue starting or running later.