upgrade operating system

There are instructions + links to get downloadable installers for current & older macOS:

• How to get old versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

Note instructions above, used to mention bootable installers; see details here:

• How to create a bootable installer for macOS - Apple Support

https://support.apple.com/en-us/HT201372

Good luck & happy trails!

🌞🌿⛵️🎣🌻🐢⛄️🌤🌎

Hello wilycayote! In order to update your iMac, you should go to the Mac App Store and search High Sierra on the search bar. You should see the icon of High Sierra now on the results!

I hope I solved your problem!

Have a great day!