This wasn't easy since the instructions for the calibration were in German. But here's a translation:
FUNCTION TEST
When you start up the following functions using the throttle stick, you can consider using the LED.
• Make sure that the controller is not connected to the drive battery.
• Remove the motor pinion, or ensure that the wheels can rotate freely.
• Turn on the transmitter.
• The throttle lever of the transmitter must be in neutral.
• Connect the control to the battery.
• Press the Setup button at least 2 seconds with the plastic screwdriver.
• The Setup - LED flashes green and neutral is automatically saved.
• Move the joystick on the transmitter first to full forward and then Full reverse.
• Release the throttle stick and go into neutral.
• Press the Setup button again to save this setting.
• The setup LED lights up solid green.