Android 4.3 update does not brings any major changes but you can expect to see better keyboard input, autocomplete support in dialler, enhanced photo daydream, and support for additional languages.
Like all other software updates, Android 4.3 update for Sprint Galaxy Nexus is being rolled-out in phases and you might not get it right-away. To manually check the update, go to Settings << About Phone << System Update << Check Now.
With Android 4.3 update, it is the end of road for Galaxy Nexus. In the Android 4.4 announcement just now, Google has revealed that Galaxy Nexus won’t be getting Android 4.4 update.
