Just saw the tweet from HTC UK
Lets start with the important bits: this is for the regular unbranded UK CID One M8. Update timing for branded/network ones are often a guess. Assume that other European countries are around the same time. Asia/India? We’ll see.
Great news! We'll be rolling out the Marshmallow update for the HTC One M8 over the next 48 hours. One M9 to follow shortly.
— HTC UK (@HTC_UK) January 18, 2016