I open Photos. Photos displays "Updating Library..." forever. used to work, before os update...

OK, now I'd have to be guessing on Photos. do you want me to do some investigating & then guess?

Oh wait...

How to use the Photos library repair tool

If your Photos library won't open, or if Photos behaves unexpectedly, the Photos library repair tool might resolve the issues.

Follow these steps to use the Photos library repair tool:

Hold down the Option and Command keys while you open Photos on your Mac.

In the dialog that appears, click Repair to start the repair process.

You might be asked to type in your user account password to authorize the library repair.

The repair tool analyzes the the library's database and repairs any inconsistencies it detects. Depending on the size of your library, this might take some time. When the process is completed, Photos opens the library.

If you repair a library that's automatically updated with iCloud Photos, the entire contents of the library re-updates with iCloud after the repair process completes.

Also...

https://discussions.apple.com/thread/7860541?answerId=31389764022#31389764022

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.

 To find out if it's system wide or user specific, try this...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?

Are there a lot of people's faces in it?

Oh yeah Face Recognition could take quite awhile.

Whoot, great news! :)