How do you change engine oil in a 40 hp mercury four stroke?

To change the engine oil in a 40 hp Mercury four-stroke outboard, first, ensure the engine is cool and level. Remove the oil drain plug and let the old oil drain into a suitable container. Replace the drain plug, then remove the oil filter using an oil filter wrench. Install a new filter, fill the engine with the appropriate type and amount of new oil through the dipstick tube, and check the oil level to ensure it's within the recommended range.