There is drain plug on the bottom of the gearbox, you undo the plug and the old oil pours out. Clean the plug and put the washer on it. If you look down the front (between the gearbox and the radiator) there is another bolt, undo this and fill it up from there.
I pretty much followed this video (for a MK2 Clio). It's the same.
But use the proper elf tranself NFP 75W-80 oil. The gearbox has brass synchro's so you have to be a bit careful when picking the oil.