Android 6.0 Marshmallow is now starting to roll out to the HTC One M9 on Sprint, according to the carrier's software update page. Though the update began rolling out today, February 5, according to the page, Sprint notes that the staged rollout may take several days to reach everyone.
In any case, once the update lands, you should find yourself running firmware version 3.41.651.3. Along with Android 6.0, you'll snag new features like the Doze, granular app permissions, and more. For now, you can check for the update by heading to Settings > System Updates > HTC software update > Check Now. And once you snag the update, be sure to let us know what you think in the comments below.
Thanks to Kyle for the tip!