** Warning: The default firmware shipped with the board has been tested and configured by the factory in working condition (factory flashed the board with tested firmware manually via USB). You will be responsible for damage (or any part of function not working) if you flash your own firmware. So you are recommended to use the default firmware which shipped with the board. Thank you for your understanding.
** BOOT: the boot pin on the flight controller is for DFU (the method to flash firmware), you need to short 3.3V and BOOT when you need to use this option. But please do not do it unless there is no choice. It could make your board inaccessible if there is any mistake. Do it at your own risk.