how hard is it to upgrade the Operating System on a Older model IMac??

It's easy assuming the Mac has updates available for it. However in order for us to be more specific we would need to know the model year of your iMac (check About this Mac) and which version of Mac OS the computer is currently running.

In addition to the Model year, it is important to post any "modifiers" to the year, like Early, Mid, or Late. For some iMacs, that makes a big difference in what macOS version it can run.

Example:

• "Early" and "Mid" 2009 iMacs can only run up to macOS 10.11 "El Capitan."
• "Late" 2009 iMacs can ru up to macOS 10.13 "High Sierra."