How to download High Sierra installer
The only way to get an older installer is to download it before you update your Mac (you may even find you have a copy on your Mac already).
Alternatively, find a Mac that's running an older version of the macOS, log into the Mac App Store with your password and download High Sierra's installer. Or ask a friend to download a copy of the installer from the Mac App Store for you.
As above, once the installer has downloaded don't install it! If you do then the installer will be deleted. You'll find the installer in your Applications folder.
Beware of downloading the installer from a file sharing site as you can't be sure that it's safe. It's safest to say: just don't do that.
In 2017, when Apple introduced MacOS High Sierra, the company removed Sierra from the Purchased list, which made it more difficult to revert to that earlier version, but it was still possible to download the Sierra installer via this link that Apple provided.Advertisement
The link still takes you to a page for macOS Sierra, but if you are running Mojave you will see an “Update not found” error message if you try to download it. We have been able to download the installer on older versions of MacOS though. So as above, find a friend who isn't running Mojave.