Samsung currently rolling out Android 6.0 Marshmallow update to it last year flagship devices Galaxy S6 and Galaxy S6 Edge in India. Earlier in a day Company makes it Samsung Galaxy S7 and S7 Edge for purchasing without pre-orders in India.
This Update weighs 1.2 GB in size, so to download this update get on to the WIFI. Now coming to the features which this update includes are Doze, advanced app permissions, and Google Now on Tap contextual search. Samsung has a few tweaks of its own too, including a new web browser, TouchWiz user interface adjustments, and more convenient quick toggles.
The OTA notification is supposed to be reaching devices in India today. Anyhow if you did not get the notification just try manually Settings » About device » Software update menu. Enjoy the update.