I assume you are meaning the oil in the gearcase.
Drain by removing the drain and level screws. They are either broad slotted screws or socket headed. and on the port side. Take care not to remove the smaller screw on the stbd side which is the pivot pin for the gearshift mechanism. ( a good idea to refer to a parts diagram or manual)
Refill with an outboard gear oil of around SAE 75W-90. Using a pump or a squeezy bottle inject the oil into the drain hole until it flows from the level hole. Refit the screws.
Use new sealing washers on the screws.
Change it at least every year. If the oil comes out milky or obviously contains water you will have to look at fitting new seals to the gearcase.
You may find a workshop manual at
http://boatinfo.no/lib/library.html#
You should also seriously consider renewing the water pump impeller unless you know it has been done recently
