It's really not much different than changing the oil on most vehicles. Take a look under the car to locate the oil pan. Once you locate that, the drain plug is at the rear of the pan. It requires a 13mm wrench. Simply unthread the plug to drain the oil and while that is draining replace the oil filter.

The oil filter is a cartridge type. When looking at the engine from the front of the vehicle, the oil filter housing is on the left near the front of the engine beneath the passenger side exhaust manifold. The cap requires a 13mm wrench to remove the bolt that secures it. Remove the cap and filter, replace the two rubber O-rings on the cap, install the new filter, and re-install the cap.

Then put the oil drain plug back in and fill the engine with the recommended amount of oil. The most difficult part for me is getting the cap back on the filter housing. Last time I changed the oil in my 140 it took me about an hour just to get that back on.