Add a platform to a preexisting app

Hi all,

I have an app that's already on the App Store, and I want to add another platform with which it will be compatible. The app is only on iPads for now, but I created a new build to allow it to run on iPhones. How do I add another platform on iTunes connect so the app is available for both iPhone and iPad? Would I have to submit it as a new app or submit it as a new version?
Thanks!