How to downgrade operating systems

Downgrading OS X

Read these links about how to downgrade OS X. They work for any version of OS X, namely, you must do an Erase and Install, what some call a Clean Install. You will need one of these three:

1. You will need a backup of the older system that you can clone to the startup volume;
2. a copy of the original installer for the older system that was downloaded from the App Store;
3. or a bootable USB flash drive installer.

See How to Downgrade macOS High Sierra and macOS Reversion- How to Downgrade from High Sierra.