WHAT IS THE VERSION OF MY OPERATING SYSTEM?

You can verify your computer's operating system by clicking on About This Mac under th Apple logo. That opens an information window with the information you want. There is also a Software Update button that you can click on to download the latest update your computer OS qualifies for. If you are running Catalina, and your computer qualifies, a Big Sur upgrade will be offered. You have given us no information as to what Mac you have, what OS you are currently running or what upgrade you want, so there is not much more we can suggest.