Why cant I update my HTC one?

My phone is an unlocked AT&T HTC one, I tried updating my phone but it says no update available. First I go to about then software updates and it says checking then it says no software updates at the time.I am still on android 4.1.2 and HTC sense 5.0. I downloaded the HTC sync manager on my computer but I can't update from there. I also tried the att software update but it doesn't work. Do you think phone repairers can fix this or I am out of luck?? Thank you in advance.