You mean like a patch? That is not possible yet in AdvInst. We have it on our ToDo list for a future version.
Currently, you can create a package for the next version of your product which will Update the previous version if found already installed, or install fresh otherwise.
But the package for the new version will contain all the files (changed or not) of the application.
For a new release, bump the product version higher and allow AdvInst to generate a new Product Code. Make sure the "Automatically upgrade..." option is checked and replace the files on disk with the updated versions.
Building now will generate a new MSI which contains the updated app. Installing this updated app over the older app will uninstall the old one.
Only the installed files will be uninstalled and updated, other files created by the user will not be deleted.