Hi
I too faced the same problem after the update i used notify and fitness to get rid of it
1. Download notify and fitness
2. Open and pair it and do the on screen formalities
3. Press on the 3 line on top left
4. Search for settings and click on it
5. Now again scroll down and look for mi band firmware version press update
6. Click on latest version it automatically downloads now click on start update
Till every process is completed/you recover you mi band
AnD
DONE
Thank you 😃😃