AT&T has officially kicked off the rollout of Android 4.3 Jelly Bean for the HTC One. The update should appear on your AT&T unit within the next couple of hours (if it hasn't shown up already). Users can download and install the entire update over-the-air using the inbuilt OTA updater function.
For carrier locked variants of the HTC One, the Android 4.3 update is more of an incremental update than for the international variants, which get a ton of new features including an updated version of HTC's Sense UI. Nonetheless, hats off to HTC for actually having pulled this one off. Last we checked, no other OEM such as Samsung or Sony have begun seeding Android 4.3 for any of their Smartphones yet (but that will happen soon enough). That's a big achievement for a company reputed for its lack of updates, or massive delays in delivering the same.