Asus has pushed out a new update to its Zenfone 2 (ZE550ML) smartphone. Arriving as version 4.21.40.311, the update stabilizes Bluetooth connections and improves ZenMotion sensitivity. Nothing else is included in the update. The update announcement says it usually takes around a week for the roll out to complete, so if the update notification hasn't popped-up on your device yet, be patient. Source | Via
Source: http://www.gsmarena.com/new_zenfone_2_update_rolling_out-news-25925.php