Google promised to keep Android One smartphones updated always with the latest Android version. Keeping the promise, they have already released Marshmallow OTA updates for bunch of Android One devices. But it may take some time to reach depending on carrier and region. However, if you dont want to wait for the OTA, you can alternatively download the official OTA firmware and sideload it manually on your smartphone. Download the Marshmallow OTA Firmware from below provided links for your phone model respectively.