Of course Sony's flagship is updating to Marshmallow (Android 6.0). When? Sony Xperia Z5 is supposed to have Android 6.0 by March (announced by Sony Mobile UK Twitter account). Other devices like Z4, Z3+ (Japan), Z4 Tablet will also get the Marshmallow update.
This forum thread discusses Sony Xperia Z5 will update to Marshmallow in March. You can join the discussion by clicking on 'Post a reply'. If you are new to AppFutura Tips Forum, you will need to sign up and create a new account.