Unplug your mower from its power source.
Tip your mower back to expose its underside and position some old newspaper beneath it. Prop it against a wall or some cinder blocks to gain access to its undercarriage.
Position a pan beneath your mower's drain hole, and then loosen the cap protecting it with your ratchet. You should be able to find the drain hole on the base of the body of your machine, above its blades.
Allow your mower's engine to drain completely before refilling it with fresh oil.