This is a quick video showing how to replace the gear on the front motor of a Syma S107G Helicopter. The process is pretty straight forward. Be careful not to clip any of the wires while working to remove the motor. You also need to remember that the led in the front is connected to the shroud so you don’t want to pull those wires out when you remove the plastic front end.
Here are links to the helicopter itself and the motor with the gear on the end.