how do update my iMac iOS

Welcome to Apple Support Community.

Using this Apple Support article as a guide, About System Information on your Mac - Apple Support, please let us know what year your Mac is and what version of macOS you are currently using.

Jack

The introduction year/season of your model iMac determines the maximum version macOS that it can run.

iMac (Late 2009) or newer can have macOS High Sierra v10.13.

iMac (Late 2012) or newer can have macOS Mojave v10.14.

iMac (Late 2012) or newer can have macOS Catalina v10.15.

iMac (Mid 2014) or newer can have macOS Big Sur v11.

iMac (Late 2015) or newer can have macOS Monterey v12.

Find the upgrade options available for your model in the Mac App Store and/or Software Update in System Preferences. Update macOS on Mac - Apple Support

Just for information, and to make it easier when you search for answers. iOS is the operating system used for iPhones, iPad OS is for iPads, WatchOS is for watches, tvOS is for Apple TV and MacOS is for Mac computers (the older version 10 variants are OSX)