Unlocked HTC One M8 (US) is getting its Android 6.0 Marshmallow update within 24 hours
It's worth mentioning that Mo Versi specifically talks about the US version of the unlocked One M8, so folks in other regions might not (and probably won't) get the update in the next 24 hours. Earlier today, we told you that the European One M8 is expected to be updated to Marshmallow in about two weeks.
Mo Versi further mentioned that the Marshmallow update for the HTC One M9 was also "coming soon." Several months ago, HTC said that both the One M8 and One M9 would get Android 6.0 starting the end of the year, so we hope the M9 won't be left behind.
As for US carrier variants of the HTC One M8 and One M9, these should be updated to Android 6.0 Marshmallow a bit later: starting early 2016.
source: @moversi, @HTC (Twitter)