Hi, I've recently changed my gearbox oil to redline mt90 and its made a great improvement to the smoothness of the change, not cheap at about £15 for just under a liter but well worth it.
Firstly and this is the important bit, make sure you can undo the filler plug before draining the old oil, it is an 8mm allen key plug right on top of the box, look down next to the battery, you will need an allen key socket and a 12 inch extension bar and probably a breaker bar as they can be quite tight.
Then with the undertray off the drain plug is another allen key plug, I cant remember if it is an 8 or 10mm, drain the old oil out and then refill via the bung on top using a funnel and short length of hose.
The oil capacity varies a bit depending where you get your info from, between 2.4 and 2.8 liters, I put 2.6 in mine and its been fine.
As with any oil change its best to do it with the gearbox warm as the oil will run out better, and its a good idea to keep your bottles of new oil inside somewhere warm to make refilling easier.