After your app is initially built using the App Wizard, you can now configure your app and design it to your preferences, eg in the basic settings or in app design and newsstand design .
However, changes to the app will not take effect until the app is built again.
Do you wish to test your app on a tablet or smartphone before going live? In this case, create a preview app and see if the app is up to your requirements. Read here how you can build a preview app.
In order to publish an app to an app store you have to build a release version of your app. See here how you can build a release app.