unable to update macOS big spur 11.3.1

Follow the advise of previous person.

If it still does not work, Restart the Computer in Safe Mode, SHIFT immediately at startup. It does a Repair Disk, clear cache files and only loads Apple Software and Fonts - so loads slowly - normal.

Safe-mode: shut down (or just reboot) and restart while holding Shift-key until a progress bar is seen (a red Safe boot should then be visible in the login screen).

Additional things to try: ethernet instead Wi-Fi, 40-50 GB free space, no 3rd party virus apps or cleaners etc.

Restart the Computer in Safe Mode, SHIFT immediately at startup.

If the download succeeds, and you get to the 15 minute alert, in my experience it then succeeds although it might take 15-20 (fast SSD) or up to 60 minutes (HDD).