You can just turn the "horn" - the plastic attachment thing that screws onto the shaft of the motor. This will change the starting position. You cannot change the maximum angle the motor turns. While the motor is capable of turning about 120 degrees, the circuit of the bit limits it to a smaller angle.
As an alternative, you can modify servo code found in the Arduino IDE's File > Examples > Servo menu. Connect an Arduino board, and a servo motor using a proto bit. This way you are not limited by the bit's circuit - you can build your own, for any size servo motor you'd like. Look at this old post to see how to hook that up.