Android 6.0 includes new features like Doze. This helps improve standby times and battery life by putting the phone into a deep sleep when it is not being used. The update also brings the ability of users to approve or deny permissions for apps on a feature-by-feature basis. And Google Now On Tap will provide search results based on content found on the screen.
Once you receive notification that the update has reached your HTC One (M8), make sure that your phone is connected to a Wi-Fi network. Also, you should check to make sure that your battery is charged to at least 50%. Remember, the update is sent out in waves. This means that your phone might not be among those that receive the update today, tomorrow, or even over the next few days. Be patient and before you know it, you will be rockin' Android 6.0 on your One (M8).
HTC's Mo Versi says HTC has received technical approval to send out Android 6.0 to Sprint's HTC One (M8)
source: @moversi via AndroidCentral