Your Mac will now restore this backup, and downgrade you to an earlier version of macOS in the process!.Choose the backup that you want to reinstate, which will typically be the last backup you created before updating macOS. The next screen will display all of your backups.On the ‘Restore from Time Machine’ screen, click ‘Continue.’.In macOS Utilities, select ‘Restore from Time Machine Backup.’.Release these keys as soon as you see the Apple logo. Restart your Mac while holding down the ‘Command+R’ keys.

If the backup is stored on an external or network drive, then make sure this drive is available to your Mac.Reinstalling an earlier backup will undo all the changes you made since creating this backup, so make sure all of your documents and data are stored somewhere safe before proceeding. If you create regular Time Machine backups (and you should!) then you may be able to reverse a macOS update by restoring a Time Machine backup that you created before installing that update.

Choose your startup disk, and then select ‘Install.’.

Once you see the macOS Utilities screen, select ‘Reinstall macOS’ and then click ‘Continue.’.

Restart your Mac, while holding down the ‘Shift+Option+Command+R’ keys.

If you’re on High Sierra 10.12.4 or later, and you want to revert to the version of macOS that shipped with your Mac, then you’re in luck! This is one of the easiest ways to downgrade your Mac:

How to update macos high sierra