The two year old Galaxy S5 is receiving a Marshmallow based update from AT&T with build number G870AUCU2DPK2. The Security patch will bring performance improvements and bug fixes which include call and data reliability improvements.
The Galaxy S5 wasn’t a revolutionary smartphone when Samsung launched it and it received a lot of criticism for its build quality. Nevertheless it did packed solid internals. On the software front the S5 was updated till Android 6.0.1 Marshmallow. Unfortunately the Galaxy S5 is not in line for the Nougat update but there are other ways to get Android Nougat running on the device.
On a side note, if you’re bored with TouchWiz on your Galaxy S5 you could root your device and flash different custom ROMs for a new feel. Cyanogenmod 14.1 based on Android 7.1 Nougat is available for the Galaxy S5.