How can I downgrade from macOS Monterey to macOS High Sierra?

Do you have a Time Machine backup to Restore to/from?

https://eshop.macsales.com/blog/32972-time-machine-what-it-is-how-it-works-and-how-to-use-it-with-your-mac/

Revert your Mac to a previous macOS version - Apple Support

If not…

https://www.imore.com/how-downgrade-macos

Yes, one possible workaround fo some newer files, but nothing automatic...

Is to get another external drive, install HS to that drive, boot from the new external drive holding Option key on startup... see if you can scrounge around & retrieve any less than year old files...

Find Any File...

http://apps.tempel.org/FindAnyFile/index.php

Hold Option or alt key when selecting Find to Find All.

In Home folder of the Monterey install.