Locate the spark plug on the lawn mower engine. The spark plug is visible and you don't have to remove any parts from the engine to access it.
Disconnect the plug wire from the top of the spark plug using a pair of pliers. Grasp the connector at the end of the wire and pull it straight upward until it disconnects from the top of the spark plug.
Place a spark plug wrench over the top of the plug until it is secure around the spark plug base. Turn the wrench counterclockwise until the spark plug is loose enough to remove by hand.