MacOS High Sierra is the latest update on my Mac Desktop. I want to restore to factory settings. How?

Hi there

See this link and follow the instructions, starting under the paragraph “if your mac does not support erase all content and settings”

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

What year model is your iMac?

see > Identify your iMac model - Apple Support

For a 2010 or earlier, use the original Install Disc set or a USB installer to erase and reinstall.

For a 2011 or later, you can use macOS Recovery to erase and reinstall the macOS.

see > What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

and > Use macOS Recovery on an Intel-based Mac - Apple Support