Unable to upgrade firmware on Vidonn X6 bracelet
Got the same issue on my OnePlus, unable to upgrade firmware, my smartband had some issues (screen not working). After long search, found it : uninstall vidonn2.0 app , download vidonn tool.apk and firmware https://goo.gl/BZYGsQ (read the text file) then install it, reboot phone, launch Vidonn tool then flash firmware. You can also try to flash the firmware in the Vidonn2.0 directory on the phone.