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:

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

How to Downgrade ASUS Zenfone 2 :

1. In this tutorial I will downgrade from 2.20.40.156 to 2.20.40.139
2. Download ADB and Install it
3. 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
4. Download Firmware (lower version than your current version) -> I download 2.20.40.139 version
5. Place Recovery.img, bootloader.img, boot.img to Downgrade Folder
   
6. 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)
   
7. Connect your Zenfone 2 to PC/Laptop
8. Type on CMD (press enter after each line):
fastboot flash fastboot droidboot.img
fastboot flash recovery recovery.img
fastboot flash boot boot.img
9. Enter recovery (Tutorial Here)
10. Select Apply update from adb
11. Open Ze551ml firmware folder and press shift+right click > open command here
12. Connect your Zenfone 2 to PC/Laptop
13. Type on CMD (press enter after each line):
adb sideload UL-Z00A-WW-2.20.40.139-user.zip
14. Factory Reset (To avoid a crash error, but not perform factory reset is ok)
15. Done