You don't say what type of engine is fitted, but there will be a drain plug under the mower on the bottom of the engine - similar to a car. It will either be a large hex bolt head, or a square drive female recepracle i the head of the plug.
Change the oil when HOT - be careful it will be as thin as water and can cause severe burns.
phil_saunders@bigfoot.com
Phil Saunders, March 2009