HTC One M8 for Windows - Manual (Page 196)

A factory reset may not permanently erase all data from your phone,
including personal information, which may be recoverable by others.
Performing a factory reset via settings
The most convenient way to do a factory reset is via the settings.
1. Swipe down from the top edge of the screen with one finger.
2. Tap All settings > about.
3. Tap reset your phone, and then tap yes.
Performing a factory reset using phone buttons
If you can’t turn your phone on or access settings, you can still perform a
factory reset by using the phone's buttons.
1. Turn the power off.
2. Press and hold the VOLUME DOWN button, and then briefly press
the POWER button. Release the VOLUME DOWN button when you
see an icon on the screen.
3. Then press the following hardware buttons in sequence: VOLUME
UP > VOLUME DOWN > POWER > VOLUME DOWN.
196 Update and reset