Hi
saijyotesh.29Thank you for your asking.
1. Auto-Update (Same Android version only)
Settings>System>System update>Check
2. Manual Update
First, check your current firmware version by the following steps below: Settings>System>About Phone>Software information
Second, check ZE551ML firmware spot and download the next version after your current one
# ZE551ML firmware spot
https://www.asus.com/Phone/ZenFone-2-Deluxe-ZE551ML/HelpDesk_BIOS/Third, put the file into the root directory, the outermost place, of the internal storage.
Last, reboot, and tap the notification "system update"