gouache
After compiling , you should see the firmware file "firmware.hex" in the folder FIRMWARE_LOCATE\Marlin\Marlin\.pioenvs\fysetc_STM32F1\
Download the flash software in our github https://github.com/FYSETC/STM32Flasher
Connect the board and your pc with USB cable (This step should before you open the software)
Double the click "FlyMcu.exe" software to open it.
Select "Reset@DTR low(<-3v),ISP @RTS High"
Click "Port" to select the port of your USB
Select FIRMWARE_LOCATE\Marlin\Marlin\.pioenvs\fysetc_STM32F1\firmware.hex as the screenshot shows.
At last ,click the "Start ISP(p)" to upload the firmware.