Update version 23.5.A.0.570 is now pushing out to the Sony Xperia Z3 Tablet Compact, which upgrades the tablet to the latest Android 6.0.1 Marshmallow OS. The update is now live for the LTE (SGP621, SGP641) and Wi-Fi (SGP611, SGP612) versions of Sony’s tablet.
This is one of the last Sony Xperia device holdouts for Marshmallow.
Source: Xperia Z3 Tablet Compact gets Android Marshmallow (23.5.A.0.570) | Xperia Blog