Hey there, afisher!
Thanks for reaching out in Apple Support Communities! If we understand correctly, you're trying to update your Mac but it's stuck installing. We'd be delighted to help get this update installed!
Have you tried forcing your Mac to shut down? If not, give this a try first by pressing and holding the power button for up to 10 seconds.
Should the update continue to give trouble, use the following support article for guidance:
If your Mac starts up to an Apple logo or progress bar - Apple Support
"If you still need help, please contact Apple Support."
Take care!