Supplemental update stuck on installation screen - how to break free?
In general, Apple software updates will try again and if you’re stuck, you’ll eventually need to power off the machine and risk leaving the disk in a bad state.

I would power down and try restarting and see how it goes. Worst case, use Internet Recovery when you can to install whatever version of OS you want. As long as you don’t install an older version, your data should all be intact. Command R is the usual command to initiate internet recovery when you hear the power on chime or see the Mac start to power up. 

Sometimes booting in Safe Mode (hold shift) can get past an install that’s staged, so that’s a long shot if you don’t want to seek hardware service or have a bootable OS brought to the mac so you can triage further.