After updating, you should see a new full-screen “immersive” mode, as well as restyled notification and navigation areas. Battery performance should also drastically improve with the update, as should general OS performance, or so says AT&T. Other new features include cloud printing, Tap & Pay for NFC payments, the ability to choose a default SMS app, colored emoji support, and improved music controls from the lock screen.
New full-screen mode
Restyled status and navigation bars
Battery performance improvements
Enhanced processing speed
The update comes in at 686MB and will begin to rollout tomorrow. You can check for the update by heading into Settings>General>About phone>Software update>Update now.