To revert your Mac back to High Sierra, follow the steps below.
Step 1: Back up your Mac
You should back up your Mac before you start any major process, and hopefully you backed up before installing Mojave. If you’re unsure how to back up your Mac, you can follow the steps in this article. However, any files you’ve used or been working on since you installed Mojave won’t be up to date on that back up, so you need to copy those to an external disk or a cloud storage service like iCloud, Google Drive or Dropbox. Don’t do anything else until you’ve copied those files.
Step 2: Make notes
The process of downgrading wipes everything from your hard drive, including passwords, license keys and settings. If you have a backup of your Mac from before you upgraded to Mojave, you should be able to migrate much of that data back to your Mac once you’ve reinstalled High Sierra. However, it’s a good idea to make sure you have a note of all the passwords, settings, licence keys and other data you’re likely to need.
Step 3: Downgrade
How to downgrade from a Time Machine backup
- Since most users back up their macOS before updating, in most cases restoration from Time Machine is simple. You can use that to reinstall High Sierra. Make sure your Time Machine disk is connected to your Mac, either directly or over a network, before you start.
- Erase your startup disk, as described below in Step 4.
- In the macOS Utilities window, choose Restore from Time Machine Backup.
- If your backup is on an external disk, select it. If it’s on a Time Capsule or network disk, select it and choose Connect to Remote Disk.
- Type in your name and password for the disk, if necessary.
- Select the date and time of the backup you want to restore to.
- Follow the onscreen instructions.
How to downgrade from macOS Mojave if your Mac shipped with High Sierra
- Erase your startup disk as described below in Step 4 — you need to do that first because Recovery mode won’t install an older version of the OS over a newer version.
- From macOS Utilities, choose Reinstall macOS.
- Press Continue.
Step 4: Erase Mojave
Once you’ve backed up the files you’ve worked on since installing Mojave, and created the bootable installer, it’s time to erase Mojave:
- Make sure your Mac is connected to the internet.
- Click on the Apple menu and choose Restart.
- Hold down Command+Option+Shift+R to boot into recovery mode. Note, you can also boot into Recovery mode by pressing Command+R. However, adding Option+Shift will allow you to reinstall High Sierra, if your Mac came with it installed.
- Click on Disk Utility in the macOS Utilities window.
- Select the disk with Mojave on it.
- Choose Erase.
- Give the disk a name, choose Mac OS Extended (Journaled) or APFS as the file format.
- Click Erase.
- Quit Disk Utility.
Step 5: Reinstall High Sierra
- Go to the Apple menu, choose Restart, and hold down the Option key.
- When the option to select a boot disk appears, choose the installer disk you just created.
- High Sierra will begin installing on your Mac.
- When it’s finished, your Mac will restart and Startup Assistant will appear.
- Go through the steps to set up your Mac.
Step 6: Restore settings
If you made a non-Time Machine backup of your Mac before installing Mojave, you can use the backup tool to restore your Mac to the state it was in when you made the backup. Otherwise, you’ll need to reinstall apps manually, using the notes you made earlier to enter licence codes and re-create settings. You can also copy back files that you backed up when you were running macOS Mojave.
The installer for macOS High Sierra can be found here: https://support.apple.com/en-ca/HT208969
Edited by Jim Wilson