what is the Update from iMac OS X 10.6.8
Your Mac is a 2009 iMac. The latest OS is can run is MacOS 10.13 (High Sierra.)

Note that "UPDATES" are different from "NEW OS versions". An update to Snow Leopard is like from 10.6.3 to 10.6.8, or a security patch. 

A New Version is a separate product, and must be downloaded from the App Store, rather than Software Updates. You may want to try this link for El Capitan.

It's likely that Apple will only show you the latest OS (High Sierra) for download. Unless you have specific needs, I would recommend installing High Sierra, and keeping all your software up-to-date. Snow Leopard is 10 years old!