The engine should be on a stand or the transom of your boat, out of the water. You should find a large, slotted head machine screw on the forward part of the lower unit, just below the propellor shaft/gear case.
There should be another slotted head machine screw above it, about 8 inches higher. Remove the top screw, then the lower screw. They will be TIGHT, so take care not to mangle the screw head. A very large screwdriver works best. Have an empty can or bucket below to catch the watery lubricant. Then, refill the lower unit with the manufacturer's recommended lubricant. This is usually done by filling from the bottom orifice (using a pressure pump) until the oil runs out of the top hole. Then tighten the top screw before removing the pump from the lower hole.