Changing oil is usually a pretty straightforward procedure.
Generic instructions:
Warm the engine up to operating temperature. Really isn't necessary but most manufacturers recommend it.
Find the drain plug(s) somewhere near the bottom of the engine, put a pan underneath and take the plugs out.
Find the oil filter. Replace it. If it's a spin-on, lube the sealing gasket and don't overtighten. If it's a cannister type filter, take note of how the original is installed and install the new filter the same way. Lube any o-rings or gaskets it may use but then wipe the oil off these. The only function the oil serves is to allow the o-ring or gasket to easily move to where it needs to be.
Install the drain plug(s).
Fill only with the correct diesel-rated oil of the viscosity recommended by the manufacturer. Usually 15w-40 but 10w-30 and straight-weight diesel oil is available, too.
Fire it up, check for leaks, and top off the oil as necessary.
That's really all there is to it, unless Yanmar differs significantly from normal.
