To change the blades on a lawn mower, empty the gas from the mower, unplug the spark plug wire, and turn the mower on its side. Loosen the bolt to remove the old blade. Assemble the new blade on the engine crankshaft, and tighten the bolt.
Tipping the mower on its side with gas and oil in the engine may cause a leak. Run the mower until it is empty. To avoid burns, wait until the mower cools before working on it. Unplugging the spark plug is also a safety precaution to ensure the motor doesn't turn on while working on the machine. Wear work gloves before handling the blade to avoid a cut.
A bolt holds the blade to the engine shaft. Choose a socket that fits the bolt to remove it from the mower. Some lawnmowers have two more bolts on the blade adaptor. You need a blade adaptor for the new blade, so remove those bolts, and set the adaptor aside.
Attach the blade adaptor and blade to the engine crankshaft. Ensure the blade goes on the same way as the old blade. Read the lawnmower owner's manual to find the proper torque setting for the blade bolt. Turn the lawnmower upright. Reconnect the spark plug wire, and refill the lawnmower with gas and oil to prepare it to mow.