HTC One M8 for Windows - Manual (Page 193)

Update and reset
How to get and install updates
From time to time, software updates may be available. Your phone can
check and notify you if there’s a new update.
Setting update notification and downloading option
You can set your phone to notify you when new updates are available for
your phone. You can also allow your phone to automatically download
those updates.
1. Swipe down from the top edge of the screen with one finger.
2. Tap All settings > phone update.
3. Select the Automatically download updates if my data settings
allow it check box. If you disable this option, your phone will only
download updates over Wi-Fi or when it's connected to your
computer.
Update and reset 193