Changing the lower unit oil is not too difficult.
Most important thing is to make sure you have the right oil per the manufacturer's manual.
Then make sure you have a good drop cloth/catch bucket and lots of rags - it is messy.
There are 2 plugs - top and bottom. On my BF50 the top plug is to the left (facing toward the bow of the boat) above the horizontal fins. The bottom plug is also slightly to the left side of the "bulb" at the bottom of the unit. The plugs will look like fairly large straight screw heads (not phillips).
Take the bottom plug out first and get your drip pan in place. You'll need the motor in the "down" position so that it drains correctly, so you either need to get it hanging over a curb or something, or you'll only have a very tight space to work with down there low to the ground. At first very little fluid will drain because the oil is thick and the top plug is still in.
Next take out the top plug and the fluid will drain faster. It is easiest to do this on a hot day when the oil flows a bit better, especially if you are impatient
Now comes the fun part: You put the new oil in FROM THE BOTTOM. I have been told you do this because
if you put it in from the top, you get trapped air pockets inside and you'll think it is full when it is not.
Your replacement oil should have come in a squeeze tube or tubes, this is easiest. If it came in a can or jug, you'll need a little pump - WM sells them as do some auto-parts stores.
Insert the nozzle of either the squeeze tube or the little pump into the BOTTOM drain hole and begins putting in the oil. Watch for it to reach the level of (and start coming out) the top hole. Once it is full, keep the nozzle in the bottom hole and insert the top plug (a helper is very useful at this point), this will seal the sump and then you can take the nozzle out of the bottom without much oil coming out down there.
Put in the bottom plug. Let the whole thing sit a while, maybe overnight if you have the time. Then check for leaks under the motor, and take out the top plug and inspect the oil level. I have been taught that it is ok to add a little bit of oil through the top hole to top off the sump if needed, but if you find yourself needing to put more than an ounce ot two in the top, you are better off taking out the bottom plug and pushing the oil in from there.
Note: While filling the sump, if you need to use more than one squeeze tube to fill it, stick in the top plug while you switch tubes to slow down the stuff running out during the switch.
Another reason there is a bottom plug, is of course that water is heavier than the oil so if allowed to settle, any water in the sump will settle at the bottom. That way you can drain a little bit out the bottom to verify you have no water contamination without draining the whole thing.
- AndyS
, it reminded me of my first experience. I thought that lowest screw was down there to change the regular motor oil so I accidently unscrewed it to drain the engine oil only to later discover I drained the wrong one. I was really Pi$$ed with myself about it 
especially when I got to the fun part of squirting it back up the same hole, sure won't make that mistake again. Well I think I got it filled but some of it drained back out while trying to get that bootom screw back in so I just put a bit more through the top, no way I was going to undo that screw again, what a mess, I wasn't properly prepared for it. 
Still learning to be a do it youself guy.