The following steps helped me for MBP 2017:
Turn off MacBook;
Hold shift, turn on MacBook, it should launch to desktop in Safe mode;
Open App Store, search "big sur" and press "Get", System Preferences should open with downloading of full Big Sur update (~12GB);
Wait until installation completes and reboot normally, you can work meanwhile, however UI is quite slow in Safe Mode.
All personal files and apps remained intact, TimeMachine restore was not needed.
Avertae
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