macos HIGH SIERRA/ MOJAVE
HOW TO DEINSTALL MOJAVE AND GO BACK TO HIGH SIERRA?
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Boot into Recovery mode, and choose to Restore from Time Machine backup — provided it was your last High Sierra Time Machine backup before the Mojave upgrade.
If you never took a Time Machine backup of High Sierra before the High Sierra upgrade, then stay with Mojave, as you would have to wipe the Mojave drive with a clean install of Hig Sierra and have no backup to recover your personal data and configuration settings.
And retreating to High Sierra eliminates any browser or security updates from Apple as High Sierra has fallen off the supported operating system list.