To change the gearbox oil, use a jack to lift the car and get underneath it. There is a duct under the gearbox that can be opened using a spanner. This will let the oil flow out. Close the cap again and refill oil from the oil cap which is located in the hood of the car.
There is a small plastic thumb screw wing nut on the side, not the top of box as that is a breather pipe. Use a pipe and fill until the oil starts dribbling out, the gearbox will hold around 3 litres on a Renault Kangoo on JC1 boxes.
Park car on a level surface, Handbrake on, unfasten locking nut under engine & drain into suitable container. After drained change oil filter, replace nut & fil with oil in 1/3 stages until same amount drained off replaced..