Hey Ed,
What kind of motor do you have?
Where do you do your boating?
Typically, if you're in a colder climate, you ideally want to change your gear oil at the end of the season before you put your boat away. If you're in a warmer climate then it becomes a maintenance item that you will perform when needed.
Basically what you do is pull the lower drain plug and then the top one and drain the oil into a pan. The top drain screw acts as a vent, so once you pull it the oil usually comes out pretty quickly. It's always a good idea to have a look when it first starts to drain and see the condition of the oil. If it appears milky, that would indictate you've got a bit of a seal leak and water is getting into your gearcase.
Once you let the gear case oil drain completely, you basically do the reverse to fill it. So you put your bottle or hose (if you have a pump) into the LOWER drain hole and start filling the case. I usually do it fairly slowly myself to make sure the case gets completely filled. Once you see gear oil start coming out the top vent hole, you'll know the case is full. You then keep your bottle or hose in the bottom hole and then put the screw in the top vent hole. Once you get that screwed in, you then have to quickly pull out your hose or bottle and quickly screw in the bottom plug.
As for what oil to use, it depends a bit on your motor. If you tell us the year and type we'll figure it out together.
