Hi, normally the fail is caused by Bluetooth re-connection fail before upgrade.
Please try:
1. delete your device from APK by drag it towards LEFT
2.research device by pressing + on top right
3. if device can be found again, please continue sw upgrade.
If above does not work, you have to wait for battery used up, and then recharge it.
The bluetooth re-connection fail is a bug from Bluetooth firmware which is coming from Bluetooth chipset vendor.