Just found this.
//Data loss during Sierra upgrade is not limited to any particular Mac machine, but it can happen on iMac, MacBook Air, MacBook Pro, MacBook Mini, etc. So Apple always recommends backup Mac hard drive with the aid of Time machine application before every OS X upgrade. The Time Machine backup will restore the previous version of OS X by recovering all the data back.//
Get rid of antivirus applications installed on your Mac machine.
Try upgrading the software in Safe mode.
Switch from wireless connection to hardwired connection.
Check whether you Mac device is suitable for Sierra upgrade.
"Time Machine" is recommended on many sites to do system recovery after High Sierra fail. I am loathe to post the link as it is not a free aid. This is free to read however.
http://www.macintosh-data-recovery.com/blog/recover-data-failed-macos-sierra-upgrade/