Changing the oil in your push mower is not a complicated job, but unless you can get it up and onto a work bench, you will be doing this on the ground. Generally this must be done once per year under normal cutting circumstances, but heavy brush cutting or large amounts of acreage mean the oil should be changed twice a year. As always, check your oil level before each time you mow, and add some if needed.
Prop the front wheels of your mower up onto concrete blocks. This will allow access to the underside of the deck.
Find the oil drain plug, which will be slightly protruding under the bottom of the deck.
Unscrew this plug with your adjustable wrench. Most drain plugs are square-shaped, so a regular wrench will not work.
Position the bowl beneath the plug, and on the last turn when the drain plug comes out, the oil will begin draining into the bowl.
Pull the blocks out from under the wheels and allow the oil to drain into the bowl with a level deck. Allow to drain for about 3 minutes.
Place the mower back on the blocks and remove the oil-filled bowl. The used oil can be taken to any gas station and most hardware stores for recycling.
Screw the drain plug back in and tighten it securely with the adjustable wrench.
Remove the mower from the blocks and fill the motor with the right oil type, per the manufacturer's directions. Check the dipstick to be sure the motor is full and ready to go.