You cannot update an existing app w/o changing its version code. Each time you update your app, you should increment its version code by 1. The version code is not externally visible to the user, and is used only as a means of versioning your application.
You can update an existing app w/o changing its version name. The version name is what the user will see on Google Play (i.e. version 1.10, 2.08, etc.), and has nothing to do with the actual version number of your application (it can be anything you want).