HTC's commitment to faster updates has seen an improvement since the launch of the HTC One A9 last Fall, and a series of older flagships including the HTC One M8 and HTC One M9 in some regions have started to see the latest version of the Android software come to them as a result. While models like the Bell Canada HTC One M9 and the unlocked HTC One M9 have already started getting Android 6.0 over the past month and a half, not all carrier models here in the U.S. have pushed Marshmallow out to phones. As of today, though, Sprint begins the process of sending out the over-the-air update to HTC One M9 owners meaning most subscribers with this phone could end up seeing the update before the weekend is over.
While there is no clear indication of how long the OTA update will take to reach every single HTC One M9 user on Sprint's network, the fact that the update is happening right now is likely to make quite a few customers happy as many have been waiting on the update since the unlocked model started receiving it in December. If you're a customer on Sprint and you haven't yet seen the update come through to your device, you can always navigate to the settings menu, scroll to about phone, and then tap the software update to see if the update can be pulled down manually. If it's ready and waiting for you, you'll be alerted as such. If not, then you'll either have to continue waiting patiently or manually checking the software update status fervently.
As for what's new with this update on the HTC One M9, Marshmallow wasn't about huge, striking visual changes, so there won't be much new to look at it in terms of design. There are, however, quite a few under the hood changes to look out for like the granular permissions that allow users more control over app permissions, and the Doze Mode which Google developed to assist users in extending their battery life a little bit. The update is likely a rather large file, so it's recommended to have as full of a battery as possible and it wouldn't hurt to be connected to WiFi before trying to download the software update package.