what is the Update from iMac OS X 10.6.8 When I go into my Apple Software Updates on my OS X 10.6.8 computer to update to OS X 10.9, the computer tells me that I do not have any new updates.

How can I upgrade to OS X 10.9 if it does not give me that option? 

Operating system of my iMac
  Model Name:   iMac
  Model Identifier: iMac10,1
  Processor Name:   Intel Core 2 Duo
  Processor Speed:  3.06 GHz
  Number Of Processors: 1
  Total Number Of Cores:    2
  L2 Cache: 3 MB
  Memory:   4 GB
  Bus Speed:    1.07 GHz
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!