If you are on Sprint and own an HTC One M8 or its special Harman Kardon edition, here is some good news: the carrier has announced the roll-out of Android 6.0 Marshmallow for the handset, and it should begin anytime now. Sprint HTC One M8 / M8 HK Owners! Great news, we have received technical approval on the Marshmallow OS update! OTA will start tonight! Thx! Mo Versi (@moversi) January 29, 2016 As usual, the announcement was made by HTC's VP of Product Management Mo Versi on Twitter. He said that they have received technical approval for the update and the OTA roll-out will begin tonight (US time). This comes around a couple of months after the Taiwanese company started rolling out the Android 6.0 update to unlocked M8 units in the US. At that time, it was said that the carrier versions will get the update "as soon as possible." If you aren't already aware, Sprint's HTC One A9 has just started receiving the Android 6.0.1...
Source: http://www.gsmarena.com/sprint_htc_one_m8_marshmallow_update_imminent-news-16350.php