Drain plugs should be the big Allen nut or standard shape nut, cant exactly remember, on the bottom of the box, and the lower one of the two holes on the diff as you look at the rear of the car, the top is to refill, i'd make sure they both undo as you don't want to be stuck with an oil less diff,
So crack the plugs off the diff and gearbox let drain, pop the lower of the diff plugs back on then fill diff with 75/90 gear oil of your choice until it comes out of the top hole, this is the level line so pop plug in hole job done, as for the gearbox refill this is the fun part, you'll need a funnel and some tubing to make life easier, fill up with your choice of oil
Look between the intercooler and turbo and you'll see the oil box dipstick, this also doubles up as the fill hole,