How do I update the OS on my early 2009 iMac? I have El Capitane now.

Can't. I had a early 2009 24" iMac and could not update. Now I have a 2019 21.5" iMac with 16mg of ram and 1 TB SSD. Not being able to update that slow 2009 iMac made up my mind to purchase the new one. So glad I did.

I have an old early 2009 iMac sitting around and generally collects

dust for the very same reason. It is now just a novelty to play with

when I have nothing else better to do and try things like install

different variety of Linux and even managed to get it up and running

with Windows 10 with a little effort.

You can not, El Capitan 10.11 is the latest mac OS that your Early 2009 iMac can run.