There are a few ways to change the oil in a Mercury MerCruiser, which include going through the dipstick tube or drain plug. Changing oil through the dipstick tube is usually reserved for those engines that do not have an accessible drain plug.
To change the oil from the drain pan, place a container under the engine and loosen the bolt. Let the oil flow out into the container, then tighten the drain bolt. Top of the engine through the oil fill hole.
Changing the oil through the dipstick tube is a little more trickier and requires the use of an oil pump. The manual pump threads into the engine via the dipstick tube hole and pumps the oil out.