how to switch back to an older operating system

You can restore the original OS that shipped with your Mac using Internet Recovery. If you want any of your data from your machine, make sure you have a backup first.

About macOS Recovery - Apple Support

If you don't want the original OS, it's possible to download other versions of macOS through the Purchased pane in the App Store. Only versions of macOS which you've downloaded using your Apple ID will appear in the Purchased pane, and only versions of macOS compatible with this exact model of Mac will actually download. Once downloaded, follow instructions here to create a bootable installer on a USB drive. Some Terminal experience is helpful but not necessary if you can follow instructions to a T. The same caution regarding your existing data applies to this method—always, always, always have a backup! How to create a bootable installer for macOS - Apple Support

macOS Reversion- How to Downgrade from High Sierra

How to Downgrade macOS High Sierra