The HTC One M8 Eye is making the long-awaited switch from Android 5.0.2 to Android 6.0.1 Marshmallow in India. The first stage of the process involves downloading a 119.7MB interim update, after which you'll be able to install the 1.07GB Marshmallow update.
Following the update, the phone's software version is incremented to 5.07.720.1, with Sense remaining at 7.0. Alongside all the new features in Marshmallow, the update includes the February security patch. If you're rocking the One M8 Eye in India, head to Settings -> About -> Software updates on your phone to manually download the update.
Thanks Devavrat Pohnerkar!