Should I update El Capitan

El Capitan is the latest OS X you can run on iMac Early 2009. Latest update = 10.11.6 released about 5 months ago. I believe you'd better off with the latest update if you don't already have it.

An early 2009 iMac can only upgrade as far as El Capitan 10.11.6. Launch the App Store and click on Updates to see if there are any available. Apple usually keeps the three most recent OS's up to date with security updates etc, since Mojave was introduced in September, El Capitan fell off that list and will no longer receive new security updates, Safari updates etc, but check on the App Store to see if any of the older ones are available for you.