macOS Big Sur 11.2.3 update hangs at 10 minutes to go

A couple of methods. Shutdown Router then same to computer. Disconnect all external drives. Restart Router and connect computer via Ethernet Cable ( Wifi is iffy ) Restart computer in Safe Mode ( Shift ) - immediately at startup. It does Repair Disk, Clears cache and only loads Apple Software and Fonts. Load slowly - normal. Try again and if possible do not let computer to go into Sleep or Screen Saver mode.

If over Wifi - same as earlier except Disconnect All Other devices using the wifi - what the Max Bandwidth for this computer.

Go have a coffee, or lunch, and give it time. As they say, a watched pot never boils...

You said 15 minutes. Now you say overnight. Of course if it is not able to complete overnight that is a problem.

Try P. Philips's suggestions.