Upgrading my ios on my imac.

iOS is the iPhone operating system. macOS is the Mac operating system.

The model year of an iMac is available at Apple menu > About This Mac.

An early 2009 iMac can be upgraded to El Capitan 10.11.6 but no later version. See: How to download macOS - Apple Support

A late 2009 through mid 2011 iMac can be upgraded to High Sierra 10.13.6 but no later version. See: How to download macOS - Apple Support

A late 2012 through late 2013 iMac can be upgraded to Catalina 10.15.7 but no later. See: How to download macOS - Apple Support

A mid 2014 and late 2014 iMac can be upgraded to the latest version of Big Sur macOS 11 but no later. See: How to download macOS - Apple Support

A late 2015 iMac can be upgraded to the latest version of Monterey macOS 12. See:How to download macOS - Apple Support

A 2017 or later iMac can be upgraded to the current version of Ventura macOS 13. See: How to upgrade to macOS Ventura - Apple Support

If you clicked the High Sierra link in How to download macOS - Apple Support the following should have happened:

1. Click the link to open the App Store page for that macOS. 
2. In the App Store, click the Get button to begin downloading the macOS installer.
3. After downloading to your Applications folder, the installer opens automatically. Follow the onscreen instructions to install macOS.

If not, see the "If you need help" section of that support article.

What is the model year of your Mac? That is shown at Apple menu > About This Mac.