OS SYSTEM QUESTION

The OS immediately following yours is Sierra 10.12. If you are interested in upgrading, you can look at instructions here: https://support.apple.com/en-us/HT208202

The current OS is Catalina 10.15. In step 4 of those instructions if you look right below where it talks about Catalina, there is a download link for Sierra.

If your iMac is a Late 2009 model, then it can upgrade to macOS High Sierra, which is currently supported by Apple for security and browser updates, and by Microsoft as the minimum operating system for supported Office 365/Office 2019 for Mac installations.

From menu : About This Mac, look on the Mac description line for the market introduction time frame in parenthesis (e.g. Late 2009). If true, so is the first paragraph. The upgrade hardware requirements are identical with those for Sierra. If that parenthetical introduction date is earlier than a Late 2009 model, then you are stuck on El Capitan.