At work we use WiX for building installation packages. We want that installation of product X would result in uninstall of the previous version of that product on that machi
I used this site to help me understand the basics about WiX Upgrade:
http://wix.tramontana.co.hu/tutorial/upgrades-and-modularization
Afterwards I created a sample Installer, (installed a test file), then created the Upgrade installer (installed 2 sample test files). This will give you a basic understanding of how the mechanism works.
And as Mike said in the book from Apress, "The Definitive Guide to Windows Installer", it will help you out to understand, but it is not written using WiX.
Another site that was pretty helpful was this one:
http://www.wixwiki.com/index.php?title=Main_Page