Hi linux, sorry for late reply. Here I share some info how I fix the the update firmware issues.
At starting, my Mitv fails update at 85% & this is due to system apps were modified by me, so I get the recovery app from "gene". To recover, I think adb tool cannot be use because xiaomi already disable the adb mode in miTV. But since u already rooted, you can find the way to enable it. But I propose another easy way: use Root Explorer. U just need copy & paste the app into the system folder & change the app permission.
After I recovered the system apps, the update failure again at 95%. So I uninstalled the king root & make it unroot state. after that I update again, wow! system can update.
If you never modified/delete system apps, you can try uninstall the root & update again. It might possible let you update the system.
If you still intend to try adb tool, here the link:
https://www.dropbox.com/s/74zavfjdxs6lx27/ADB Tool.rar?dl=0
Good luck!