Good news this morning for those of you who have the Samsung Galaxy S6 Active from AT&T. The U.S. carrier has released the Android Marshmallow update for the device and it is rolling out now. The update is build MMB29K.G890AUCU3CPE4 for those keeping score at home and it weighs in at a hefty 1.44GB. That means you will most certainly want to download this via Wi-Fi which, depending on your network, could take 20-30 minutes to download. Once you have it downloaded, the installation of the update will take another 20-30 minutes and will require a phone restart. Make sure you have any local files (docs, photos, etc) backed up to your favorite cloud service just in case things so pear shaped.
Once the update is installed, you will have all of the benefits of Marshmallow on your Galaxy S6 Active. The update does take you to build 6.0.1 so you have the latest update to your phone and this
update has the June security update so you are pretty much up-to-date there too. In addition to all of the features that come in Marshmallow, AT&T themselves have enabled Wi-Fi calling on the S6 Active. This feature, which the carrier has enabled on several of their phones, allows you to make calls over Wi-Fi (VoIP) instead of using your mobile minutes.
The update should be coming to devices over the course of the next several days. You can manually force a check by going to Settings>About Phone>System Updates and tap the Check for Update button. If it is available for your phone, it will start downloading at that point.