Stuck on updating MacOs (BigSur) on MacBook Pro M1

Workaround is to boot Mac into 'Safe' mode (hold down shift key when turning on until the Apple Symbol appears) - login in (you will see safe mode in red on the menu status bar) - then proceed with the update.

The Safe Mode booting will automatic perform the Repair Disk and clear Cache Files etc. There is not User Interaction required for this to happen.

As for Third Party software interfering - Any of these will fall into this category - AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation. The Built-in Security of Big Sur is all that is required.

Agree with @ASH_IT - Safe Mode is best bet. It will do a Repair Disk, clear Cache File and Only load Apple Software and Fonts. This eliminates Third Party Software and extensions from interfering with the update process. It may or may not allow the update process to complete successfully.

So far I personally (and one other user) have only experienced this issue on a MacBook - not an iMac which last time updated without safe mode. I will be installing 11.3.1 after work on the iMac and will see if the pattern continues.

Are there pre-existing Unresolved Software Issues. They should be resolved prior to upgrading - they may carryover to Big Sur making the system unstable / unusable. Notation: Using Migrated from Old to New may have brought any existing problems from Old to New.

VPNs - required are on these Forums usually for two purposes. Connecting to Banks and if Required by HeadOffice for working off-site. Most Commercial VPNs Purport to keep your Data and Usage Secure and Private - the Jury is truly out on that.

It is likely to be the Anti Virus (endpoint security) affecting the update - although this has only occurred for me on the last two updates - booting to Safe Mode will disable this along with other processes during the upgrade. VPN software is unlikely to affect the update if not connected at the time and presumably a split tunnel is used to direct only 'work related' traffic via the VPN with the remainder following the normal route and in any event your 'download completed' normally.

Next time try the update with your AV installed first - if it fails uninstall the AV and try again, if that works then you know it's the AV software for sure. You always have 'safe boot' as fall back.