Sony is not known for updating their devices to recent versions of the Android operating system as fast as other manufacturers, but they keep giving support to a quite good amount of devices. For Android 6.0 Marshmallow, they selected some European users with a Xperia Z3 or Xperia Z3 Compact so they could install a conceptual build based on that version of the OS in order to test it for bugs or issues. This way, the company could fix some of them based on the feedback they got from those users. The software was somewhat unpolished, as it featured an almost stock looking version of Android with a few apps from Sony. This build was updated very frequently to add more features and users were quite pleased with the improvements that Marshmallow brought to their devices.
While we know which Sony devices will get updated to Marshmallow, we still don't know when the company will be releasing the final build for any of them. Yesterday, a few images from a Japanese carrier suggested that the company could be close to releasing the final version of the software and they even gave us a first look of the software. Now, Sony has released an app called Xperia Beta Program, further suggesting that the final version of Android Marshmallow for these devices is almost ready. The app is available for devices such as the Xperia Z3, Xperia Z3 Compact and Xperia Z2, but it is only available in a few regions including Italy, Spain and the Netherlands. The intention is letting applicable users test a pre-release version of the software so that Sony can release the final build to many more devices without as many errors.
On the other hand, the Concept build will continue to work with the devices that are currently running it and even though it was reported that users would have to uninstall it in early 2016, new information suggests that a new update for that version is coming within a couple of weeks. We still don't know how much Sony has changed the whole interface now that it is based on Marshmallow, or if some of their apps will get any new features, but apparently (and hopefully), users won't have to wait much longer.