There are two types of Update Installations available for the InstallScript project in InstallShield.  The Full Release and Differential Release types.  Both are used to update an existing earlier version of a product.  There is one critical thing you want to remember when doing this.  You want to always install the update to the same folder as the original product is installed in.

For example, if your original product is installed in the C:\XYX Company\Super-Duper-Product folder, then when you define the update installation, either a Full Release or Differential Release, you want to make sure it is installed to the same folder.  In this example, the update would be installed to the C:\XYX Company\Super-Duper-Product folder.  If you don't do this, you will get inconsistent results, and pull on your hair, and say horrible things while you are trying to figure out what the problem is.