A safe way back to 'stock' firmware for hackers, and an alternative way to update
In a somewhat unusual move, HTC has published the new Android 4.3-based firmware for the T-Mobile HTC One online, in the form of an RUU (ROM update utility). The RUU weighs in at just over one gigabyte, and can be used to directly flash the latest "stock" firmware onto T-Mo's HTC One, regardless of whether it's running older official software or a custom ROM.
In the past HTC has shied away from releasing these kinds of utilities publicly, likely due to the software licensing restrictions, though that hasn't stopped many RUUs from leaking out online over the past several years. If you've got a T-Mo HTC One and want to perform a "manual update" as HTC calls it, you'll need a Windows computer, the correct drivers, and a little patience and technical know-how.
You'll find the download link and instructions below.