1) Download and save on PC your device specific firmware from
ZenFone 6 (ZS630KL) BIOS & FIRMWARE | Phone | ASUS Global2) Connect ZenFone 6 to PC with USB cable
3) Transfer (drag and drop) firmware file (xyz.zip) from PC to the root of ZenFone 6
4) unplug the USB cable from phone
5) A notification should come up on phone to install firmware. Just follow the instructions.