There isn’t any changelog for the updates, however we can assume that Google’s built-in features, like Now on Tap, Doze mode, and more flexible app permissions are all present in the Marshmallow updates. There’s also a new version of TouchWiz that’s present alongside other improvements specific to Samsung and T-Mobile users probably won’t notice.
To get the update, you must either wait for the notification to be delivered to your phone telling you that there’s an update available for download or check manually by going to Settings > About phone > Software Updates on your device. It may take a while for the update, which weighs 1.2GB, to reach your Galaxy device, so be patient. I suggest being on Wi-Fi during the downloading process and having your phone plugged in during the installation so a) your data doesn’t get gobbled up and b) so you don’t run into any errors during the installation process.
Did you receive Marshmallow yet on your Galaxy S6 or S6 Edge? Let us know in the comments!