Best way is to remove the drain plug at the bottom of the leg. It will be buried in crud and needs a perfect fitting screwdriving and probably an impact one. Refill with engine oil and replace the seal on the plug.
I can't stress how important it is to use a "perfect fitting screwdriver." The plug is made from relatively soft stainless steel and the shoulders of the screwdriver slot will become rounded if the screwdriver you use is worn and also if it is too small. If you use an impact driver, be very gentle with it as the sail drive leg is an aluminium casting, and an over generous impact might result in needing a new sail drive leg.
In the past when I did not have the right screwdriver for the job, I used my engine oil extractor pump with a thin tube attached to suck the old oil out via the saildrive dipstick plug hole on the top. This needs to be done when the oil in the saildrive is warm, i.e after a run of half an hour or so.
Also, in my opinion, sucking the oil out as described above should not be a long term solution, as you may not get out the last drops of oil that contain any impurities.
A further point is to put the old oil into an empty and dry water bottle which you then leave to stand for a day at least before you dispose of the oil. The oil as it comes out should not be mushy which would indicate that you have a bearing seal leak, and leaving it to stand will allow any water in the oil to settle to the bottom, thereby making its presence very clear. If you have a leaking seal, and unless you are very competent, it is something best left for the yard engineer to fix for you. Apart from that, the oil change is otherwise a pretty easy job to do, but DON'T, repeat DON'T overfill the sail drive when the new oil is put in. Overfilling if not corrected will result in oil being expelled into the engine compartment when the leg warms up while in use and the oil expands.
