You can now download un official version of Android 6.0 marshmallow on your Sony Xperia Z2 tablet. This ROM is based on AOSP Android M build and is very easy to install. Everything except camera is working on it and since not a many of use cameras on our tablet devices, it would not bug us much. Sony is taking quite a lot of time to push Android 6 marshallow for the Xperia Z2 tablet users and fortunately, thanks to folks at XDA, Xperia Z2 tablet users will be able to download android 6 on their tablets. 
Installation instructions.
1. Flash this Boot via Fastboot (don’t use boot.img from ROM) Castor-Windy Image
2. Boot to Recovery ( Power Off device than restart it and while the LED lights Purple press Volume UP and Down some times)
3. FACTORY RESET and FORMAT SYSTEM (needed if you have LP or other custom LP Roms)
4. Download ROM Castor-windy
4.1 Download ROM Castor
5. Flash SYSTEM.img and BOOT.img from ROM via FASTBOOT
6. reflash first Boot.IMG
7. Boot to recovery
8. flash gapps zips http://opengapps.org/
9. reflash Boot.IMG Form Rom
10. Reboot and try it
To attain root on Xperia z2 tablet, download this file and reboot to recovery (like in STEP 2) than flash SUPER-SU.zip. Then boot to fastboot and flash back BOOT.img from ROM. Xperia Z2 tablet specs.