https://support.apple.com/en_US/downloads/ios

What model iMac do you have and what version of macOS is it running?

click the Apple on the menu bar and select "About This Mac"

iMac desktops do not run iOS, they run macOS.

see > Apple - Support - Downloads

Go to the App Store and the latest system that your Mac can run will be available to you, probably High Sierra if it's a late 2009 or newer.