Updates

Start with knowing the market introduction date in parenthesis found on the product description line from menu :About this Mac. Once you have that, check if the iMac meets the hardware requirements for the following. Use only the Safari browser to visit these sites:

How to upgrade to macOS High Sierra (late 2009 iMac or newer)

How to upgrade to macOS Mojave ( only if the iMac is a late-2012 model or more recent)

How to upgrade to macOS Catalina (only if iMac is a late-2012 model or more recent)

How to upgrade to macOS Big Sur (only if your iMac is a 2014 or later model)

How to get old versions of macOS.

Imacs have been available for over 20 years, did you expect us to know which one you own???