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You could break the blade on your lawn mower accidentally, just by mowing the yard. There are often things hiding in the grass, unseen because the grass has gotten to be too long. These could include toys that your children or your dog left outside, branches that fell off of the nearest tree during a wind storm, or rocks that were pushed there in the winter by the snow plow. Hitting these things when you thought you were just going to be hitting grass could cause the blade to bend or even snap.
The first step to replace the blade is to make sure that you have the right lawn mower parts. Your manual that came with it should tell you what blade you need and even give you a parts number so that you can order it. If you no longer have the manual, look it up online and it will not be hard to find. Putting the wrong blade on could be disastrous, so make sure that you have the right one.
After that, drain the gas out of the mower and turn it on its side. You may want to disconnect the spark plug so that there is no chance that the mower can start. You will see where the bolt holds the blade in place. Just take this bolt off - the setup is different for different mowers, so be sure that yours fits this style - and then put the new blade on and refasten it.