How to Downgrade macOS High Sierra
Some MacOS High Sierra 10.13.x users may wish to downgrade back to macOS Sierra 10.12.x or even Mac OS X El Capitan. Mac users can downgrade from High Sierra to a prior Mac OS release, either by formatting the hard drive and clean installing Sierra or another prior system release, or by relying on a Time Machine backup made prior to the update to macOS High Sierra.
The downgrade method we’ll cover here uses a Time Machine backup to restore to a prior version of macOS and downgrade macOS High Sierra 10.13. If you do not have a Time Machine backup made with a previous version of Mac OS, this will not be possible to follow.