While Sony's latest crop of devices, the Xperia X series, come running on Android 6.0 Marshmallow out of the box, older devices like the Xperia M4 Aqua and the Xperia M5 have been stuck on Android 5.1 Lollipop. That changes today as Sony has just started to push out the software updates that move those two devices forward to Android 6.0.1 Marshmallow, bringing them in line with the Xperia X series as well as other older Sony smartphones like the Xperia Z5, Xperia Z3, Xperia Z2 and others. While the updates have started to roll out to both of these phones, it's important to remember that it is possible that not all owners of these models will see the update today.
Having said that, if the notification alert has not popped up for you, you can always check for the update to the software manually by heading into the settings menu and then about phone, where you can tap on the updates button to see if it's available to pull down from the network. Alternatively, you can also hook your phone up to the Xperia Companion PC software that Sony has made available and see if the update shows up for you that way.
As this is Android Marshmallow, Xperia M4 Aqua and Xperia M5 owners can expect to see the usual changes that have come along with this Android version, two of which are geared towards better battery life - Doze Mode and App Standby. In addition to these two new traits, Google Now on Tap is also going to be integrated, as well as more granular permissions control allowing users to grant permissions to apps after the first time they open them up to use instead of immediately upon trying to install the app. The new permissions also allow users to grant permission at a granular level, meaning they can go into a specific app from the settings menu and tap on the "permissions" section where they can choose to grant some but disable others, although it should be noted that this can cause the app to not work properly. It's also worth mentioning that the Xperia M5 model which has received the update is limited to units with an E5603 model number, while the Xperia M4 Aqua update is being rolled out to most variants including the Dual-SIM model.