Place a drip pan or other container on the side of the mower with the filler. Carefully tilt the mower so oil drains out and into the pan. Discard the spent oil into a proper disposal vessel. Now slowly pour fresh oil into the mower. For the specific type of oil you need check your instruction manual. Make sure to add only as much as your particular model requires. Remember to let the mower sit undisturbed for a few minutes so the oil settles properly inside the engine.
For the past three years we have used the Greenworks 22502 Corded Lawn Mower on our ¼-acre lawn. We particularly like how easy it is to adjust the blade height and it is surprisingly light. There are just a couple negatives we have noticed. We live in the Pacific Northwest where mowing wet grass is unavoidable. When the grass is wet it does not feed into the bag of this mower easily. Also we had to replace the cord after my teenager ran over it with the mower. So cord control is important and takes a little planning.
Over 2500 buyers have left four- or five-star reviews of the Greenworks 22502 on Amazon. The most helpful reviewer compared this model to a couple Black and Decker electric mowers he had owned in the past. He was much more satisfied with his Greenworks mower because it could handle wet grass and was easy to maneuver.