The Samsung Galaxy S6 active, which was launched as an AT&T-exclusive in the US last year, has finally started receiving the Marshmallow update. Carrying a build number of MMB29K.G890AUCU3CPE4, the update brings along Android version 6.0.1. Weighing in at around 1.4GB, the update also includes the WiFi calling feature, in addition to the usual Marshmallow goodies such as Doze, Google Now on Tap, and redesigned app permissions. As is usually the case with OTA roll-outs, it might take some time for the update to hit your device. Meanwhile, if you feel impatient, you can manually check...
Source: http://www.gsmarena.com/samsung_galaxy_s6_active_starts_receiving_marshmallow_update-news-19227.php