Galaxy S5 not updating Android system

Hi, newbie here.
I have a Samsung Galaxy S5 unlocked AT&T. Provider is Straighttalk. The Android system has not updated beyond ver. 5.0 (not even 5.0.1). Here are numbers that may help provide a solution.
Ver. 5.0
Baseband ver. G900AUCU4BOF2
Kernel ver. 3.4.0-4432708
Please tell me how to get the phone to update to the most recent version.
Thanks, thelonius

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