Sony is beginning the rollout of a new firmware update for its current flagship, the Xperia Z2. The software (version number 17.1.2.A.0.314) doesn't bump the underlying version of Android, which was already based on KitKat. Instead, Sony is cleaning up the rough edges.
Here's the changelog for the 17.1.2.A.0.314 firmware.
Enhanced voice call quality
Improved touch function and performance
Charging improved when battery is low
Overall performance and stability improved
And all of our latest bug fixes and improvements
As Sony is fond of pointing out, software updates happen at different times in different regions, so you might not be getting an update just yet.