It would appear that most of the newer Android devices are starting to get their share of the latest version of Android with version 6.0 (also known as Android M or Marshmallow). It would appear that Sony is getting its consumers on track with the latest version, as the Japanese company is often one of the slower rollouts in comparison to Samsung, HTC, Motorola, or LG. This is the current news about Android M Version 6.0 Marshmallow Release Date for Sony Xperia Z5, Z4, Z3, and Z2 on Sprint, T-Mobile, AT&T, US Cellular, and Verizon.
Sony is on the cusp of introducing its consumers to the Sony Xperia Z6, which should have Android 6.0 right out of the box. Except the name of the Z6 won't probably happen, as Nashville Chatter reveals that it has been announced that the Z5 will be the last in the series. Perhaps the movement to the Xperia A, Xperia XA, and Xperia X Performance (also preloaded with Marshmallow) represents a new move for the company, but what is in the cards for Sony's older Android smartphones?
From what it sounds like, the rollout for the Xperia Z5 series which includes the flagships of the Z5, the Z5 Premium, and the Z5 Compact has already happened, according to Android Pit. In fact, this Marshmallow rollout has been available as of March 7, and was announced on Sony's Xperia News Twitter Page. The issue with the rollout is it is "staggered" with different locations receiving the update at different times. There was a report of a bug in the initial rollout phase, but there has been little news of that, so it looks like Sony has ironed out that particular issue for their devices.
There have been reports that the Xperia Z4 Tablet should have Android M, but most are concerned about the still popular Xperia Z3. The International Business Times reports that the Marshmallow upgrade has been made available for the Sony Xperia Z2 as well as different variants of the Z2 tablet as of last week. This goes for the Xperia Z3, Xperia Z3 Dual, and Xperia Z3 Compact, so it should be available, but this might vary depending on your carrier.
Other Sony products that are set to receive the Marshmallow upgrade include the Xperia M4 Aqua, the Xperia C4, and Xperia MS, but there have been no release dates confirmed. The Z1 series is definitely not going to receive the upgrade, and that includes the Xperia Z1, Xperia Compact, Xperia Z Ultra, and the Xperia Z.
So basically, if you have a Sony Android device that is relatively new, then you should go into the "Settings" and "System Update" to see if the upgrade is available. As stated before, the upgrade may not be available depending on your carrier like Sprint, T-Mobile, US Cellular, AT&T, and Verizon.