Sony's Xperia Z1, Z2 Compact and Z Ultra can get an update with the Android 6.0 Marshmallow via an unofficial ROM, which means that it was developed by the Android community.
According to Neurogadget, the AOSP ROM has only a camera bug which affects the Z Ultra and Z1 Compact phones, while all other features such as the Networks and Calls, Wi-Fi, Display, Brightness, Bluetooth or Orientation and Ambient Light Sensor, don't have any problem.
For users owning any of the three Sony handsets, search on the internet for the Android 6.0 Marshmallow AOSP ROM for the model, then get the Google Apps and the SuperSU root package from Chainfire.
Before installing the ROM, make sure that the battery of the device is fully charged or at least 80 percent full, so it won't shutdown during the installation process.
Now, prepare to install the Marshmallow ROM, which was downloaded from the internet, along with Google Apps. Create a folder on the computer with the name "update6" which will contain these four files: system.img, cache.img, boo.img and userdata.img, then copy to it.
The last step is to install Google Apps and then return to the TWRP recovery home screen, then tap on Install and after locating the Google Apps file, just tap on it and swipe again from the bottom of the screen. The app package starts to flash and when Google Apps completes installation, restart the device by tapping on the Reboot system button.
Android Origin said getting an unreleased Android update via the custom ROM route can be possible, as a matter of fact, this was the first way to get ones hands on the Android 6.0 Marshmallow update even if having a Nexus, as official or unofficial images are the most basic way of changing the OS.
Android 6.0 Marshmallow update image is now available for Sony Xperia Z1, Xperia Z1 Compact and Xperia Z Ultra, and it is reported that it works pretty well.