Google Now On Tap comes with Marshmallow, offering a contextual search experience. Search replies are based on the content on your screen at the time of the request. And app permissions are handled on a feature-by-feature basis. For Galaxy S6 users on Sprint, the update also comes with a new version of TouchWiz, which makes Samsung's UI more usable.
When you receive the notification revealing that the update has arrived on your phone, make sure that you are connected to a Wi-Fi signal before you proceed. At the same time, check to make sure that your battery is charged to at least 50%. Finally, because the Android 6.0 update is being sent out OTA in waves, it might take a few days before you actually receive it. You can try to coax the update out manually by going to Settings > About phone > check for update.
Take a gander at a couple of screenshots taken from a Sprint branded Samsung Galaxy S6 in the process of being updated, by clicking on the slideshow below.
Thanks for the tip, Federico!