iMac upgrade

Is this an Early, Mid, or Late 2009? Those are different models with different max OS limits. We can more easily help you if you let us know that all-important sub-model, from "About this Mac"

The Early and Mid 2009 iMacs can only upgrade to macOS 10.11.6 "El Capitan."

The late 2009 iMac can go up to macOS 10.13.6 "High Sierra."

See: How to get old versions of macOS - Apple Support

What happens when you use Software Update?

The Mac App Store.

Also, please never post personal information including but not limited to:

·Serial numbers

·Phone numbers

·Full names

·E-mail addresses

·Physical addresses 

when posting on a public forum.