The Sense 6 update being pushed out to HTC One (M7) owners is a whopping 695MB. If the update is similar to the one that was just sent out to the unlocked and Developer Edition versions of the HTC One in the U.S., it will include a number of features including new a user interface for both the Gallery and the Camera applications. It should also include the Extreme Power Saving mode. The update will take the software version on your phone to 5.14.531.1.
The Extreme Power Saving mode is also the new feature being sent out to T-Mobile's HTC One (M8) users. With this feature turned on, your phone has functionality limited to making/taking calls and texting. HTC says that the Extreme Power Saving mode turns your phone from a power-guzzling smartphone, to a tea sipping handset that uses 1% of battery life every three hours. Your battery life is supposed to increase by 40% with EPS mode turned on. The update also adds improvements to Wi-Fi performance and the audio quality of the phone. It brings the software version on your HTC One (M8) to 1.57.531.7.
Since each update is sent out on a staggered basis, be patient. The appropriate update might not hit your phone for a few days. In the meantime, you can try to coax it out of your device by going to Settings > About > Software updates and then check to see if the update has arrived on your device. Whenever the update arrives, make sure you are on a Wi-Fi connection with your battery charged at least half way before you download and install it.
source: T-Mobile (1), (2) via AndroidandMe