Sony says "The only way to find out if there is an available Android 6.0 (Marshmallow) update for your Xperia™ M4 Aqua or Xperia™ M4 Aqua Dual is to connect your device to a computer running the Xperia™ Companion application.
There will be no notification in your Xperia™ device announcing the Android 6.0 update. Nor will there be an indication in the Software update app under Settings > About phone > Software update that an Android 6.0 update is available.
Software updates are released at different times depending on the operator, market, region or country. Therefore, we cannot give you an exact date for when a specific update will be available for your device. In addition, network operators and specific markets may choose not to avail of the updates for specific models."
Mine has 6.0, there was no notification and even when I checked or an update in settings it didn't come up. You have to connect it to a computer with the Xperia Companion on it