To replace a Toro lawnmower's spark plug, first remove the boot cover on top of the spark plug, and remove the spark plug wire from the spark plug. After this, connect a spark plug socket to a ratchet, and then set the plug socket on top of the spark plug.
Rotate the ratchet's handle in an anticlockwise direction to the point you feel the spark plug turning, and then finish removing the plug by hand. If the spark plug doesn't easily turn, don't use force as this can damage it. Instead, spray its base with oil, wait for a few minutes, and then try turning it again with the ratchet.
Check the manual that came with the Toro lawnmower for information regarding the correct size and type of spark plug. Also, check for the right measurement for the spark plug gap. Install the replacement spark plug to the port where you removed the previous plug, and then start turning it slowly in a clockwise direction by hand. Attach the ratchet with the plug socket to the spark plug, and continue turning the replacement plug firmly into place. Afterwards, reattach the spark plug wire, and then slip back the boot cover back into place.