There should be a drain screw at front bottom edge of box and a screw at halfway split section on the side ! some have a screw in behind the prop to drain so remove prop and see , once drained then refill through the drain hole until it comes out the other side screw ! P...

Take out oil drain screw at bottom of gearbox and vent srew above cavitation plate,drain oil,squeeze in oil from bottom till it comes out vent hole,while still holding oil bottle in position screw in vent screw then screw in drain screw