I have a iMac 10.4.11. How do I upgrade/update? Thanks

The first step in Upgrading... is to Snow Leopard = OS X 10.6.x

It is Not available as a download... It is a Paid Upgrade.

Do this first...

Check that your Mac meets the System Requirements for Snow Leopard...

Snow Leopard Tech Specs

http://support.apple.com/kb/SP575

If so Contact Apple in your Country to Purchase a Snow Leopard Disc...

http://support.apple.com/kb/HE57

In the US...

1-800-MY-APPLE or 1-800-676-2775

After the Successful Install, run Software Update to get the latest updates for Snow Leopard.

Be sure to make a Backup of your Current System Before Upgrading...

The first thing we need to do is find out which model iMac you have. iMacs span over 10 years and just "iMac" isn't very informative. Some can only go to Leopard (10.5), others can got to at least Snow Leopard (10.6) and maybe beyond.

Get more information about your computer. Go to the Apple in the upper left corner of any window, then "About This Mac". Write down what it says about "version"and report that here. Now continue to "More Info..." Copy and paste the information here, but omit the serial number and Hardware UUID (if present).

http://www.everymac.com/systems/by_capability/maximum-macos-supported.html

iMac G3, iMac G4, iMac G5, iMac CoreDuo, iMac Core2Duo ?