Remove the eight gearbox screws with a #2 Phillips screwdriver (note that on older versions, the screws will likely have hex sockets, and require a 2mm hex driver instead of a Phillips).
Flip the gearbox over so that the front is pointed to your right. Insert a small tool through the back of the gearbox to help hold the spring guide rod down when the gearbox is opened.
Place your thumb on the cylinder as shown, and hold it down. Using your index finger, carefully pry the gearbox open from the top while holding the cylinder and spring guide down.