The Android 6.0 Marshmallow update is going live starting today for Galaxy S6 and Galaxy S6 Edge devices. The update will arrive for other Galaxy devices before long, too. If you have an upgradeable model, stay tuned for schedule announcements on that front.
If you don't see the update now, check later in the day and over the next few days, if necessary.
6.0 features battery life optimization, fingerprint security, an improved interface, edge screen enhancements, and more.