According to HTC's Product and Service Director, the HTC One M9 official Android 5.1 Lollipop update should be live now for Europe, the Middle East, and Africa.

Android 5.1 coming to EMEA HTC One m9 devices tomorrow...enjoy

 graham wheeler (@wheelergd) August 3, 2015

The update OTA package size will be about 831MB. In addition to all the new features in Android 5.1, the update improves the amount of time taken to charge the battery.

HTC is rolling out the update in stages, so if you don't see the OTA just yet, hold tight. It's coming.

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