Android 6 Mashmallow is available unofficially for the Samsung Galaxy Tab S. This Android 6 is available for these models – T700, T800 and T805. No T705 yet. The ROM is based on AOSP which means no Samsung bloatware and so it runs very fast.
You will need TWRP for Galaxy Tab S in order to flash it since this is a custom ROM. Also you must have earlier upgraded to stock Android 5for Tab S as this Marshmallow requires bootloader upgrade. Installation instructions are given below.
Flash TWRP 18.104.22.168 or higher using odin.
Reboot into recovery. Go into wipe menu. Select "Advanced Wipe". Check
and wipe them. Don’t reboot just yet.
Go into install.
Flash GAPPS.zip (tbd)
Flash SuperSU.zip (tbd)
Copy SuperSu and Gapps zip to /sdcard/OpenDelta/FlashAfterUpdate so that they are automatically applied after OTA update. Keep in mind that Camera and fingerprint sensor are not working.