[ZenFone 2 General] baudrate76:
So this is my experience, I'll share my details first.

1. Z00A- WW version

2. TWRP 2.8.7.2.

3. Rooted Method #3

4. Bootloader Unlocked (unofficial)

5. Was currently running CM 12.1

6. I have a 64gb Micro SD installed. This makes it easy for backups, restores and .img files to be stored on.

What put me on this journey was my brother's phone, which was identical to mine, with the exception of CM 12.1, he was still on Stock Rom 2.20.40.97. If you don't meet 1(may apply to Z008, I just don't have one to test), 2, 3 (either way you root is fine #3 is just what I used) and 4 conditions, then this probably doesn't apply to you.

You don't have to worry about unrooting, since we will be sideloading the factory firmware image file, you will lose root access and have to re-root the device anyways.

Before I begin anything I always do a full backup using TWRP. Even though I can rarely do a full restore via TWRP, I can always do just a Data only restore and get all my apps and files back that way. This comes into play at the end as well. So DO A BACKUP FIRST!

TWRP handicap notes

This is just my experience with TWRP, I'm a noob, have probably 20 hours invested into figuring this stuff out. I will say, I don't have any of these issues on my Samsung Galaxy S3 using TWRP, so it might be ASUS Zenfone 2 specific issues. I have been tinkering with all kinds of area's for over 20 years.

1. Restoring a full stock rom backup can and will (for me anyways) always kill the SIM 1 completely. No data or calls function, won't even detect that SIM 1 is installed, doesn't matter if Stock Rom is rooted or unrooted when it's backed up by TWRP. SIM 2 will detect my SIM card and work for phone only by it's own factory limitations (No DATA, NO 3G/4G/LTE).

2. Even restoring a custom rom has been known to cause this or at least cause no cellular data to occur on SIM 1. (you won't notice this until you disable your wifi or travel outside of the wifi your on.) It seems I'm always crossing my fingers here.

3. ADB sideload functions don't work at all, might just be for this phone, but none the less I haven't been able to sideload at all using TWRP. Only the original recovery works for me. This is why you need the recovery.img file stated below.

Steps to apply the update that worked for me.

Obviously, you must have adb and fasboot along with Asus drivers for your phone installed as a pre-requisite to doing this: if you haven't found out how to do that yet then read the resources all in one thread post located here

http://xdaforums.com/zenfone2/general/asus-zenfone-2-flashing-recovery-mode-t3096596

1. download the 2.20.40.139.zip file from asus here http://dlcdnet.asus.com/pub/ASUS/ZenFone/ZE551ML/UL-Z00A-WW-2.20.40.139-user.zip

2. download the recovery.img for 2.20.40.97 from here http://www.mediafire.com/folder/setyy42t2cymy#dy87k1a0f8m81

3. This is easier if you have a MicroSD card installed for your backups and recovery.img file to be copied to via your PC. Not necessary though, if you don't have a MicroSD card or OTB USB, then you can do the following instead;

A) Copy recovery.img to your internal root area of the phone.

B) Power off phone.

C) Boot into Recovery Mode. Holding Power btn and UP vol btn at the same time until phone vibrates, then release Power btn, then use Down vol btn to choose recovery mode. The phone will reboot to TWRP Recovery.

D) Choose Install, change to images, choose the recovery.img file copied from step A and choose recovery partition to apply to then swipe to flash.

3A. If you do have a MicroSD or OTB USB then you can just flash from this directly in TWRP recovery.

4. After Step 3 is complete, you can go ahead and wipe everything through advanced wipe, choose all except MicroSD if installed.

5. Once the recovery.img file is installed and all data wiped, you can reboot to bootloader using TWRP, you will receive a notice that no OS is installed, that is fine, you are rebooting to the original recovery tool.

6. Upon reboot you will be at the fastboot menu. Choose Recovery Mode, the phone will reboot to the original recovery mode.

A) You will hit the infamous dead robot with Error! underneath it, just hold up and hit Power btn and you will now see the original recovery menu again.

7. Now you can do a sideload, by choosing update via adb the phone will move into sideload abilities, and you can directly choose the new 2.20.40.139.zip file that you downloaded from Asus in step 1.

A) If adb wasn't installed system wide, then make sure you copy your 139 firmware file into your adb/fastboot directory.

B) Open command prompt and navigate to your adb/fastboot directory or open the directory through file explorer and hold Shift key and right click, and choose open command prompt here.

C) Type "adb sideload UL-Z00A-WW-2.20.40.139-user.zip" without the quotes, or rename it if you wish so it's shorter file name.

D) Wait for it to finish. Can take awhile, 15 mins or so.

E) Once done, reboot. This can take a bit too. You will notice a slight screen shade shift during boot. This is what you want.

F) Once booted, you will notice that you are now running .139 firmware, and you didn't have to load .90 or .97 first.

8. Re-activate developer mode again and USB debugging.

9.Re-root phone, I used method 3 again and it worked fine.

10. Install TWRP and Busybox via the Play store again. Then download and install TWRP through the app. There are other ways to do this as well, I just see this as the quickest and easiest for me. You may get an error saying that TWRP is already installed, that's fine, just acknowledge it and move on.

11. After TWRP is installed, Reboot to Recovery. You will now have TWRP recovery back. Here you can Restore your data using the Backup you created before we began.

12. Only choose data, leave all others unchecked. Restore and Reboot when finished. Phone will upgrade all your apps. Then you will have google play app updates as well and your done.

As I said earlier, I'm a noob to cellphone rooting and roms. There may be other fixes that I don't know about. But thought I would share my experience based on my phones features that I chose to do after I bought it. You are responsible for your stuff, not me, use this method at your own risk

Good luck, happy testing!

Baud