If you can't shut it down, you may have to wait for it to run out of power and then leave it off and charge it a bit (not fully). Once it has a small amount of charge, try starting it up. If it freezes again, then let it run out of power again and charge some. Then when you try to start it up this time, try doing so in "Safe Mode". If it will work without freezing in this mode (it will have limited access in that mode) then there is something on the device that is the problem. Something you have installed may be conflicting with the update, or it could be a bad app, virus, malware, etc. You will have to find the issue and remove it, then start the device as normal and see if it works.
To get into "Safe Mode" please do the following.
1. First turn off the tablet.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.