We reported earlier that AT&T and Verizon already started to deliver the Android 6.0 marshmallow to the HTC one M8 device in US. So US users already updated to Android 6.0 Marshmallow via Verizon and AT&T, as the company, HTC, gave Technical approval to them. US users already received the firmware update back in December, now the Marshmallow is completely available as official for HTC one M8.
Tough the update is currently available only for T-Mobile and Verizon HTC One M8 devices and the other HTC devices like One E9, One ME, One M8 EYE, One E8, Desire 826, Desire 820, Desire 816 and Butterfly 3 are scheduled to receive the latest firmware. But unfortunately AT&T is still waiting for technical approval from HTC. Once they get the approval , they will roll out Marshmallow update for those device mentioned above. Till than please wait.