HTC recently announced that owners of the M8 and M9 on AT&T were finally getting their upgrade to Marshmallow. That's great, but what if you haven't gotten the OTA yet or you modded your phone? Well, now there are RUU files for manual updates.
The RUU for the M8 is 1.6GB and the M9 RUU is a whopping 2.2GB. Unlike waiting for the OTA, the RUU completely resets the device. You'll lose anything not backed up, and you'll have to set your phone up all over again. You should probably grab the OTA if possible.
HTC One (M8 & M9) AT&T owners! Thank you for your patience. Marshmallow OS is approved, OTA to start tomorrow!!
HTC has done reasonably well updating its phones to Marshmallow, but AT&T's certification and testing are notorious for taking ages. Some other versions of these devices were updated in early 2016. If you feel like you've already waited long enough for the OTA, feel free to grab these RUUs. You'll need a computer and USB cable to install.