How do I force restart iPad with volume buttons while update is frozen?

You can’t do it with just the volume buttons, you also need the side button:

• Press and quickly release VOLUME UP

     then

• Press and quickly release VOLUME DOWN

     then

• Hold the SIDE button until an Apple logo appears (15-30 seconds), ignoring "slide to power off" if it appears

The operative word here is QUICKLY; and try it a couple of times, the timing is a little tricky. If done correctly this will always work unless the phone has a hardware problem. 

If your iPad is unresponsive, connect your iPad to its Power Adapter and allow the iPad to charge. Then, while still connected to external power, perform a forced-restart:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button; do not release the button until...
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

1. Press and hold the top button and the Home button at the same time; do not release either button until...
2. When the Apple logo appears, release both buttons.

If iPad doesn’t turn on, or if it gets stuck during start up, see the Apple Support article If your iPhone, iPad, or iPod touch won’t turn on or is frozen. Or if your iPad isn’t working correctly after you restart it, see the iPad Support website.