If your lawnmower doesn't have a drain plug, you have two options: either use a vacuum oil plug or tip it over. The vacuum oil pump usually costs no more than $90, and you can extract the oil through the dipstick tube. But if you don't want to spend the money, you can tilt the lawnmower over. This isn't ideal, as it can result in hard starting and blue smoke. And there's no guarantee that it'll spill out of the old oil. But nevertheless, it's an option if you don't want to spend money.