Updating MacOs and system freezes

Attempting to update my iMac to Big Sur. Download is completed and the OS starts to install. Install freezes with “less than 1 minute remaining” . Waited for an hour shutdown machine and rebooted. I get the apple icon and the the install progress bar and the message “ about 29 minutes remaining” install appears to continue but again gets stuck with 1 minute to go..Tried to start in safe mode… power down, restart iMac and press shift key.. I get the apple logo the beginning of the progress bar and then screen goes blank and then the apple logo comes back with the beginning of the progress bar this last sequence repeats. If I let go of the shift key the iMac goes straight to the 29 minutes remaining installation screen.

really need help here