Can I update the OS in my IMac 20 inch (early 2009) to anything more recent the OS X Yosemite please?

Follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

The iMac (20-inch, Early 2009) model can be upgraded to OS X El Capitan 10.11.6. Here are some articles that may help.

• How to get old versions of macOS - Apple Support
• OS X El Capitan - Technical Specifications