The Samsung Galaxy Tab S 10.5 is about two years old now, so it's a bit of a surprise that both Samsung and AT&T are still supporting it, yet alone pushing a pretty major software update to the tablet. Today, the carrier began pushing Android 6.0 Marshmallow to the tablet. Obviously this isn't the latest version of Android, but it is the next step from Android Lollipop. However, AT&T didn't stop at just bringing Marshmallow to the Galaxy Tab S 10.5. They also brought NumberSync support to the tablet. Which means you can take calls on your tablet as well as text messages, using the same phone number that is registered with your smartphone.
Now Android 6.0 Marshmallow brought many new features when it launched last year. But many of these you won't see on the Galaxy Tab S 10.5 due to the skin that Samsung has on the device. But you will get Google Now on Tap, which allows you to search for anything on the screen by simply pressing the home button and holding it. Many refer to it as Google Now on steroids, which is mostly correct. On top of that, Doze is also available in Marshmallow. And Doze actually is a bigger deal with tablets, as we don't use them as much as smartphones. But Doze will allow the Galaxy Tab S 10.5 to go into a deeper sleep and stay there longer. Which in-turn will provide your tablet with even better battery life than before.
The update to Android 6.0 Marshmallow is a pretty hefty 1063MB file. Which means you're going to want to be sure that you are on WiFi before you download it. Additionally, you'll want to be sure that the tablet has at least 50% battery left before installing it. This way your tablet doesn't run out of juice in the middle of the update, which could brick the tablet. If you haven't already received the update notification, you can head into Settings > About Tablet > System Updates and Check for Update. Keep in mind that AT&T only allows you to manually check for an update once every 24 hours. So if you aren't able to pull it down today, you may have to wait until tomorrow to get the update.