General advice follows. I may be preaching to the choir. Apologies in advance.

Did take a backup before running the update? If not, now would be an excellent time to reboot to externally hosted media and get what you can off the storage before proceeding any further.

If you have a backup, be ready to format and install the new OS from scratch then repopulate necessary files from that. You might not need to if you’re lucky and a reboot and retry all goes to plan, but still something to bear in mind if it hangs again. And maybe a third time.

Also be ready to have to reinstall the old OS if this is a case where the new OS and the old hardware refuse to get along.

Old man ramble: Back in the old days, it used to be possible to tell if a computer was doing something because the HDD would make noise, but with SSDs that’s all but impossible to do. HDD/SSD lights on the case sometimes give strong hints that something is happening, but, in my limited experience, they didn’t always match up one-to-one with what a HDD was doing, so I assume the same is true with SSDs. Onion on belt, etc.