Gearbox oil change

Which transmission? If it's the 5 speed manual there is a drain on the bottom of the differential housing, The fill is toward the front of the car up on the side of the transmission. They use a very large hex key. To refill you need the car level, then you fill the oil up until it is level with the fill hole threads. Should take about 2 liters or so. The 6 speed is similar. Be sure to use the correct fluid. While filling it's a good idea to rotate the tires a few times so the oil gets into the rear of the case. Then check it again.