One of the key factors in the life of your rotary mower is maintenance. You'll usually change the oil and air filter during the early summer. Removing the blades might be necessary as well if you want to have a nice, sharpened surface or if the blade has been damaged. After you have done it once, you will realize how easy it is.
Remove the sparkplug from the mower so there is no chance that it might start up while you are playing with the blade. This is done simply by pulling it out of the socket with a little side-to-side wiggling while you pull straight out. Leave it dangling on the wire to connect when you are done.
Tip the mower on its side with the air filter side on the top. If you do it the other way, oil will flow into the filter, soaking it and keeping it from starting later. Check the gasoline and oil cap before you tilt to make sure they are on tight. Now look under the mower to see the blades.
Place a wrench over the nut holding the blade in place and give it a good push to loosen it. Sometimes after months of use, the nut can become corroded and hard to remove. If you spray a little lubricant over the bolt and let it sit for a little, it should make it easier to remove when you try again.
Unscrew the nut and slide the blade off, straight down. Make sure you set the nut in a safe place so that when you are ready to replace it, you know where it is. Replace or sharpen the blade and put it back on in the same way as you removed it. Tighten the nut well.