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Open Utilities -> Startup Security Utilityĭetails about Startup Security Utility are explained at Apple's support site.Reboot into Recovery Mode by pressing Cmd- R.Just go get the installer from the Apple support page.If you have a Mac with a T2 Security Chip, it is easy to downgrade to the any OS Once you are booted with the installer, you can either erase your disk and perform a fresh installation of macOS High Sierra, or partition your disk and install macOS High Sierra in parallel to your macOS Catalina install. Once you have the installer ready, restart your Mac with the removable media plugged in, press and hold the Option key, and select the installer on the boot screen.