After AT&T, now US Cellular has also started rolling out the Marshmallow update to Samsung Galaxy Tab S 10.5 units on its network. The update bumps the Android version to 6.0.1, and also brings along security patches for the month of October.
As is usually the case with OTA roll-outs, it may take some time for the update to hit your device. Meanwhile, if you feel impatient, you can manually check for the update by heading to your tablet's Settings menu.