But as we said, HTC One (M8) units in North America did not have the feature, that is, until now. The FCC was concerned about the ability of the phone to make emergency calls in the low power mode. But the FCC is now satisfied, and the Extreme Power Saving Mode is making its way to U.S. and Canadian versions of HTC's new flagship phone, via a 149.85MB update. To get to the update, you need to go to Settings > About > Software and see if you can grab the update manually.
Besides adding the Extreme Power Saving Mode, the update also comes with some goodies for the camera and the gallery. The new build number is 1.55.631.4, and HTC suggests using a Wi-Fi connection to download and install the update.
The HTC One (M8) in the U.S. and Canada, is receiving an update that includes the Extreme Power Saving Mode