To force a shut down – press and hold the power button for ~10 seconds.
Then try a safe boot again. Press the power button to restart the Mac. When you hear the startup chime, press and hold the Shift key until you see the Apple logo and progress bar. You may need a wired USB keyboard for this, so plug your wireless Apple keyboard into the Mac with its charging cable, or you can even borrow a keyboard from a pc friend but the Windows key will = the Command key.
Once booted into safe mode, see if you can coax the upgrade to completion.
If not, see if you can finally get the Mac to reboot again normally.