Download STM32CubeProgrammer from ST website  https://www.st.com/en/development-tools/stm32cubeprog.html

ArduPilot firmware:  https://firmware.ardupilot.org/

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Make sure DFU driver has been installed. Check out “Install DFU Drivers” in this page

Connect the FC USB to computer while holding the boot button in.

1. Select USB
2. USB1 and click Refresh
3. Connect
4. select “Erasing & Programming”
5. Click “Full chip erase”
6. Browse the hex or bin file from your computer, if flashing ardupilot, use “ardu*_with_bl.hex“.  (MUST BE “with_bl”, bootloader)
7. Click “Start Programming”
8. After programming,  unplug USB and plug USB back in. FC will boot and run in normal COM mode.