upgrading operating system

Depends on your iMac model and what macOS it is currently running?

see > https://support.apple.com/en-us/HT201634

To check for macOS updates that are compatible for your model,

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

Possibly, it depends on what year Mac you have.

Please supply details as given in About This Mac by accessing the Apple icon at screen upper left. Leave off the serial #.

Latest OS eligibility: https://support.apple.com/en-us/HT212551