I had this happen to my drone to. And it destroyed the drone. I was flying the battery to 25% and have it go home but in my case it went from 25% to 6% and landed into a tree chopped around and flew out smacking the street which snapped the camera off and shattered everything else. Nothing was...
Unplug your cable!!! This same thing happened to me. I was connecting the USB cable from controller to drone during the whole upgrade. It was an act of trial and error that I discovered that disconnecting the cable completed the upgrade.