Hi there are a few steps:
Click Modify, then document.
switch the width and the height around.
Then change the iPhone Os Settings. On my version of flash it is located on the right of the screen under the publish tab, located in the properties section. press the edit button. You'll see a few options. The icons tab let's you change your apps icon. You'll need to fill in the deployment tab to be able to publish or test your app for the app store. (to become a developer go to developer.apple.com) The general tab lets you change file name, and your app name (pretty important), the aspect ratio (set it to landscape if you want). If you check fullscreen it will run your app without the bar at the top which displays charge, time etc. If you check auto orientation, Your app will run in both landscape or portrait, depending on what the user chooses.
Happy to help