Asus has released a software update for the ZenFone 5z, which upgrades the Android security patch level to May 2019. Additionally, the new firmware for the ZenFone 5z also improves the sound quality while bringing in some system optimizations. Asus ZenFone 5z The new build comes bearing version number 90.11.162.72 and is rolled out in batches to units with different serial numbers. If you don't see an update prompt on your device in a week or two, you can check for it manually by going to the Settings > System > System updates menu of your device. Source
Source: https://www.gsmarena.com/asus_zenfone_5z_may_2019_security_patch-news-37539.php
