OEM have been working hard to keep their devices updates with the latest OS. Recently hTc has started the rollout of Android 6.0 to all the hTc Butterfly 3 users in Taiwan. The update is sized 197.34 MB. The update is taking Rom to v1.21.709.1.
— Paul Ellison (@Paul_Ellison24) March 28, 2016
There were many improvements specified in Android 6.0 like the users will be getting unlimited Google photos cloud storage, Google Maps will now work offline. Google has already started working on Android N.
To check for update you can go to Settings>About Phone>Software Updates>Check for update.