HOW TO KEEP AN OLD IMAC (2009) STILL FUNCTIONAL?

I will try to answer as best I can. I have an old late 2009 iMac, in addition to two newer ones. It works fine. i use it primarily for Zoom calls. I did add some memory. Its operating system can not be updated any further. Many newer apps will no longer work on it. Eventually it will fail, so I will not use it for important functions. I would only use a computer like this for limited, specific purposes such as emails or surfing the internet.

Your question is a little vague.

Does your 2009 iMac startup and run?

If it startup and runs, then keep using it until it doesn't.

If it does not startup and run, then it is not cost effective to repair it.

What are you expecting, if it works now, it should continue to do so until some part of its hardware fails.