If you have a Time Machine backup with Yosemite on it you can restore from there.
Or in App Store / Purchased, do you have a download option for Yosemite?
If you have a Time Machine backup with Yosemite on it you can restore from there.
Or in App Store / Purchased, do you have a download option for Yosemite?
macOS Reversion- How to Downgrade from High Sierra
How to Downgrade macOS High Sierra
If you do revert, I'd use Setup Assistant to restore your data. This process takes a while, so do it when you won't need the computer for several hours, based on my experience.
How do I downgrade from High Sierra to Yosemite?