downgrade MAC OS

macOS Reversion- How to Downgrade from Mojave

You will need to download the version of macOS you want to install. That may prove difficult to do unless you are booted to an even lower version. Then you need to make a bootable installer on a USB stick or another drive. Boot from the installer, erase your main drive, then install macOS onto the newly erased drive.

You can make a bootable USB stick to install using these free programs which will do all the work for you. Boot the computer with the option/alt key held down and select the USB drive. Erase and install as Kappy posted.

Bootable USB Flash Drive – Diskmaker X      Disk Creator   or use Apple’s method       Create a bootable installer for OS