Please check if your phone is connected to the Internet stably and try to update again. You may update to the latest firmware version by checking in Settings > System > Advanced > System updates. Furthermore, you could try manually update the firmware.
Manually Update Guidehttps://www.asus.com/support/FAQ/1011546
ZenFone Live Firmware Spothttps://www.asus.com/Phone/ZenFone-Live-ZB501KL/HelpDesk_BIOS/
If the problem still appears, please tell us your current firmware version, and show us an error screenshot.