First identify if the tractor runs on gasoline or diesel, the motor's oil capacity, recommended motor oil "weight" and the type and size of its oil filter. The service or basic user's manual provides this information.
Locate and loosen the oil drain plug using the correct wrench size, and drain the oil into a pan of adequate size. Wipe the drain plug, and check for an O-ring washer or sealer that might need replacing. Armed with another drain pan, remove the oil filter, and pay close attention to the rubber gaskets. Coat the new oil gaskets with fresh oil, and prime the engine. Reinstall the oil filter, and tighten the drain plug, making sure to fix the O-ring washer until it seats. Next to the dipstick, locate the oil fill hole, and using a funnel, refill the motor with the specified oil.
In neutral with the parking brake on, run the engine, and let it idle for about 30 seconds. Check the oil filter and drain plug for leaks. Verify the oil level using the dipstick. Dispose of the old oil by taking it to a service shop that accepts oil for recycling. Do not dispose of old oil by pouring it into a storm drain.