Problems loading Big Sur 11.2.3 ?

If all else fails try the following;s

1 - boot into Safe Mode by booting with the Shift key held down.

NOTE: Safe Mode boot can take longer than normal as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

2 - go to the App Store and search for Big Sur.

3 - download the full installer which is larger, ~12 GB.

4 - run the installer (located in the Applications folder).l

Try updating while in Safe Mode.

How to use safe mode on your Mac - Apple Support