How to change privacy level of a Facebook post?
I'm afraid that the native Facebook app is very limited in functionality. While it has improved (quite a bit) from its initial launch, there are still a lot of things you can't do in the app.

You may be able to do what you need from the mobile-optimized website (http://m.facebook.com) but most likely you're going to need to hop over to your desktop and use the full website in your desktop browser.