Android 6 marshmallow is now available for Samsung Galax Tab Pro 8.4 LTE android tablets. Samsung stopped supporting this tablet just after an year after its release but folks at XDA have been churning updates for this tablet from quite some time and today the company has released Cyanogen Mod 13 for Galaxy Tab Pro 8.4 LTE. Only two things are not working at the moment: Auto-rotate is kind of buggy and proximity sensor not working. Here are the installation instructions: 
Do a NANDROID backup and back up all of your important files.
Wipe System, Cache and Dalvik Cache
Flash the ROM
Flash Gapps (optional)
Wipe Cache and Dalvik
Boot