Sony has officially announced that Xperia Z2 will get the new Android Marshmallow update. But there is no word about the date. Few days before news came that Sony Xperia Z4 and Z5 Japanese spotted in the Wi-Fi certification database with Android 6.0 Marshmallow. This is the indication first Z5 and Z5 will get the update in January after these devices Z3 will get and this will take lots of time.
So if you do not want to wait for the update and want to use the Android Marshmallow features than here is the custom ROM developed by rcstar6696, senior member of the world renowned XDA developer forum. It is based on Android open source project (AOSP). It brings the battery saving feature, new security boot feature and other value-added features enhance performance and user-experience on Xperia Z2. You can also follow the steps to install this ROM from XDA developer forum.
You can Download the ROM Here. But before you start installing the ROM on your phone do check the bugs as stated by rcstar6696, developer on XDA.