Use Time Machine to back up your data to an external drive.
Before downloading any files, verify that your Mac natively supports macOS High Sierra. If your device is too new (built after 2018), it cannot boot this operating system natively. or newer) MacBook Air (Late 2010 or newer) MacBook Pro (Mid 2010 or newer) Mac mini (Mid 2010 or newer) iMac (Late 2009 or newer) Mac Pro (Mid 2010 or newer) macos high sierra download dmg install
Before you begin
The most reliable approach for most users is to obtain the DMG file from a trusted source, mount it to extract the installer, either run the installer directly for an upgrade or use Terminal to create a bootable USB drive for a clean installation. When encountering the "damaged installer" error, remember to disconnect from the internet and set back the system date before proceeding. Use Time Machine to back up your data to an external drive