HTC has previously said that the smartphone should receive the update before the end of 2015. It seems that HTC will be able to do so, and thereby to provide a high refresh rate of at least parts of its devices.
Android 6.0 Marshmallow can come for the smartphone HTC One M8 in the next couple of weeks. About it reports a trusted source of leaks, LlabTooFeR via Twitter. However, note that this date called for Europe. When you upgrade the smartphone will also receive a new version of proprietary shell Sense. Now it will be 7.0 (although Sense 7.0 planned to distribute even earlier, along with Android 5.1).