Today Sony announced Xperia Android Marshmallow Beta testing program for Xperia Z2,Z3 and Z3C as well.
To enroll into Xperia beta program you need to download app from Google Play store. By signing up, you will be able to try a pre-release beta version of the Android 6.0 (Marshmallow).
Sony wants feedback from the tester before official release of Android Marshmallow for Z3 ,Z2,Z3C so Sony will release bug free version.
According to app description:Use this app to sign-up for the Xperia Beta Program for Z3 (D6603), Z3 Compact (D5803) and Z2 (D6503). By signing up, you will be able to try a pre-release beta version of the Android 6.0 (Marshmallow). Most importantly, you will also have an open channel with Sony Mobile’s developers. Your feedback will enable us to release even better software for your Xperia device. The amount of users for this activity is limited, so hurry up and sign-up!