How to Downgrade ASUS ZenFone 2 Firmware

Yesterday ASUS gave us recent update in ZenFone 2 ZE551ML 2.20.40.156 and ZenFone 2 ZE550ML 2.20.40.135. Personally I don't like this update because Now I can't use third party Themes, which you must download from Theme Store for free/paid themes from ZenUI designer and I don't see any good themes except Speed Themes :). This tutorial is applicable for Downgrading Firmware or even back to stock rom. Please do it carefully and DWYOR (Do it with your own risk) because wrong move may cause the device brick :)

Resources:

ADB Fastboot Installer (Download Here if you don't have it)
ZenFone 2 Original Recovery.img, bootloader.img, boot.img (ZE551ML | ZE550ML)
ZenFone Firmware (ZE551ML | ZE550ML) <-select OS Android and download your firmware

How to Downgrade ASUS Zenfone 2 :

In this tutorial I will downgrade from 2.20.40.156 to 2.20.40.139
Download ADB and Install it
Download Recovery.img, bootloader.img, boot.img (lower version than your current version)->I download 2.20.40.97 - must lower than 2.20.40.156
Download Firmware (lower version than your current version) -> I download 2.20.40.139 version
Place Recovery.img, bootloader.img, boot.img to Downgrade Folder
Go to Fastboot Mode (Turn off device and press power+volume up) or conncet the device to pc/laptop at downgrade folder press shift+right click -> open command here and type adb reboot bootloader (need usb debugging activated)
Connect your Zenfone 2 to PC/Laptop
Type on CMD (press enter after each line):

fastboot flash fastboot droidboot.img
fastboot flash recovery recovery.img
fastboot flash boot boot.img

Enter recovery (Tutorial Here)
Select Apply update from adb
Open Ze551ml firmware folder and press shift+right click > open command here
Connect your Zenfone 2 to PC/Laptop
Type on CMD (press enter after each line):

adb sideload

UL-Z00A-WW-2.20.40.139-user.zip

Factory Reset (To avoid a crash error, but not perform factory reset is ok)
Done