High Sierra won't install on mid-2010 iMac

Sierra was working great on my iMac, but a website I need to use wanted me to update Safari.

So, I checked compatibility on the Apple website, and it said I could update from Sierra to High Sierra.

I downloaded High Sierra from Apple, and the installation seemed to go fine. The iMac restarted several times, and the progress bar was moving along fine. But then it stopped at about 85% finished, and it wouldn't budge. I left my machine on all night, just in case it decided to finish, but it wouldn't.

The next morning, I turned it off, then turned it on, and only the blank white screen with an apple and progress bar stuck at 85% appeared.

I have an old backup (from 2016), but I don't know how to get it onto the iMac.

I'm not very technical. I ought to have googled first before attempting this upgrade, but I've never had a problem in the past with any updates on any of my other Macs, all of which are newer.

Can anyone suggest a solution? I can't start it in Safe Mode either. It just stays at the white screen with the apple logo and stuck progress bar.

Thanks!