Hi
ranierdelmar19 Thank you for your asking. Please refer to the link below
ASUS ZenFone Zoom (ZX551ML) - SupportHave you ever rooted or unlocked your device before, causing FOTA stopped?
You may update "WW" firmware version to the latest Android 5 version then update to Android 6.
#[Zenfone] How to update firmware on my phone?
1. Auto Update (Easier Way)
Settings>System>System update
*Tap the 3 dots on the top row of "System update">Auto-update settings, for related settings
2. Manual Update (Normally be used in cross-Android firmware update or rooted/unlocked device)
Download the firmware file into the root directory, aka the outermost place, of the internal storage>Reboot>Tap the "system update" notification
*To perform cross-Android firmware update, please backup the device data first.
https://www.asus.com/support/FAQ/1009748/