I downgraded a 4-year old MacBook Air from 10.13 High Sierra to 10.12 Sierra this past weekend.I wish I could go back to the last OS...there is nothing good about High Sierra......
I just downloaded the Sierra installer from the Mac App Store, created a bootable USB flash drive, booted from that, wiped out the Air's SSD, then installed Sierra. Very simple, no fuss whatsoever.
Nothing prevents you from installing an older version of macOS/OS X that your computer is qualified to run.
Last edited: