Usually, the case consists of 3 parts.
- A bottom cover
- A middle bit
- A top cover

The first step would obviously be to remove the screws that hold the case together. Remove the top of the casing and remove all the gears and axles there. This is the easy part to move between casings. Next step would be to remove the bottom cover, exposing a small circuit board.
This cisrcuit board is probably attached via some wires to a small electric motor and some other wires to a position sensor. Removing the motor and the sensor is different, depending on the type of servo and its construction. Usually you can remove them by pushing them down from the top of the case (e.g. push down the gear of the motor) and it will come out. This may take some force.