Sprint's version of the update (build number G920PVPU3CPB6) is the same as the one that has rolled out to the unlocked Galaxy S6. Google Now on Tap, Doze, and an updated TouchWiz interface are among the highlights, and the update also changes the vibration pattern for when you get calls and messages. As noted in the changelog, there is no decryption option in Android 6.0, so you might want to decrypt your phone (from the Settings » Lock screen and security » Other security settings menu) before installing the update or risk getting stuck with an encrypted device for its lifetime.
You should be getting a notification for the Marshmallow update on your Galaxy S6 sometime in the near future, and you can also check in the Settings » About device » Software update section to begin downloading the 1235.87 MB update immediately. If you have already upgraded to Android 6.0.1, let us know your experience in the comments below!
