I have an IMAC 14.3 system preference is that the highest it can go?

Are you referring to an iMac (21.5-inch, Late 2013) with a model ID of iMac14,3?

If so, the highest version of macOS that it can run is macOS 10.15 (Catalina) – which is no longer one of the "most recent three" that get security updates from Apple, or love from the likes of Adobe and Microsoft.

If you're running something earlier than Catalina, and you update to Catalina,

• 32-bit applications will stop working
• iTunes will go away, replaced by separate applications: What happened to iTunes? - Apple Support

In addition,

• You will be able to run the current version of the Firefox browser: https://www.mozilla.org/en-US/firefox/new/. This may come in handy since Catalina's version of Safari isn't getting updates any more.

Note the App Store likes to hide old versions of macOS by default, so to get Catalina, use Safari, and go through the Catalina link in this Support document:

How to download and install macOS - Apple Support