Go to Settings > System > System Update and check for update if there is an update it should pop up or if the update is just released then it might take some days to appear.
If you can't wait for update then download the file from current link -
https://dlcdnets.asus.com/pub/ASUS/ZenFone/ZS660KL/UL-ASUS_I001_1-ASUS-17.0240.2012.65-1.1.228-user....After downloading go to file manager and move or copy the compressed file from downloads to internal storage. Go to settings and check of update using the above steps, if nothing happens then restart, update notification will appear 5-10 second later and then update.
Note: Check for update or restart the device after copying or moving the file into the internal memory without any folder otherwise the update notification will not appear.
