It's been a few months since Google rolled out Android 9 Pie, which means that many OEMs with skinned UIs are gearing up to start updating their devices too. Samsung began its process with the S9 and S9+ on Christmas, and now Asus is following its footsteps with its Zenfone 5.
If you have a Zenfone 5 (ZE620KL), you should keep an eye out for an update notification. The OTA is being sent out now, though the rollout will happen in batches, so you'll have to be patient — or jam the check for updates button regularly and hope for the best.
The OTA pushes the device to firmware version 16.0610.1812.58 and updates the security patch level (though it's not clear which month). Given past updates, you should expect the size to be over 1GB so you better be on an unlimited and fast connection to grab it.