How can I replace gear oil

With the vehicle safely lifted up on a ramp, looking either underneath the transmission or on the side of it, you should find a drain plug. Typically they look like round circles with a square in the middle.

There could be two. The drain plug that sits the highest is a fill plug and the lower one is a drain plug. But some Peugeots such as the 106 fills from the very top of the gearbox.

The fill plug on the side of the gear box is a guide as to the fill level of the oil. As soon as the oil starts to dribble out of the fill hole when refilling, then that is the amount of oil required for that gear box. However it is always best to check technical data for the vehicle so you have the correct amount of gearbox fluid.