[DISCLAIMER] I AM NOT RESPONSIBLE IF YOU BRICK YOUR USDX+. I AM SHOWING WHAT WORKED FOR ME.
This video shows the firmware update process for uSDX+ using an Arduino UNO, where to get the firmware and how to update the firmware.
Original : https://github.com/threeme3/usdx
Forked : https://github.com/coffeedev/usdx_plus