HTC One (M8 & M9) AT&T owners! Thank you for your patience. Marshmallow OS is approved, OTA to start tomorrow!!— Mo Versi (@moversi) May 11, 2016
If you recall, back in April, Versi had revealed a "slight delay" in the roll-out, saying that they expect to receive the approval for the update within a couple of weeks. So, in that context, the update roll out is bang on time.
However, given that fact that three of the Big Four carriers in the US have already rolled out the update to both devices, AT&T is no-doubt late to the party.
how to updated marsmallow to HTC M7 and is there availability to get updated marshmallow
HTC isn't good in terms of updates. Samsung is even worse
What about the fact that the Samsung Galaxy S5 hasn't been updated to Marshmallow yet either? It's only now being pushed out.