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Many people here don't know how to roll back or update their phones' firmware, and that's why I'm making this guide. I hope that it's easy to understand for everyone.

HOW TO UPGRADE/DOWNGRADE STOCK OFFICIALLY

To do this process you do NOT need an unlocked bootloader, and it will NOT void your warranty. If you have an unlocked bootloader, it will also not lock it.

What you need:

-Your phone model (NOT THE MODEL LISTED ON THE FIRMWARE). Flip the phone and look where the ZTE logo is at, it should say either A2017, A2017U or A2017G.

[IMG="Back"]https://www.dropbox.com/s/6w9zsz7ix34cv06/IMG_20170828_111520.jpg?dl=1[/IMG]

-Stock recovery. If you didn't tamper with the device you'll most likely have this. If you have TWRP, you can use a flashable zip to get the stock recovery back. There are some for the A2017G, don't know about the others. You can find them at the Download Center. I recommend you to use the same recovery that your firmware version should have, for instance if you are on B04 use the one for B04 or maybe B03. But usually you shouldn' have trouble if you use an older one.

-The firmware that you'll install on the phone. You can either download it on a PC or on the phone itself, but you'll probably need to unzip it.

I recommend trying to download ot on the phone first. If it fails to download, the safe bet is using a PC though.

-RAR if you'll download the system on your phone, or an unzipper on your PC if you're gonna do it from there (7zip, winrar)

IF YOU ROLL BACK, ALL OF YOUR DATA (INTERNAL STORAGE INCLUDED) WILL BE WIPED. BACK UP EVERYTHING YOU CONSIDER IMPORTANT!

If you update, this usually does not happen, but always back everything up just in case.

Regardless that you download it via PC or phone, you'll need the system zip. Knowing the HARDWARE MODEL of the phone, download it either from the official ZTE pages, or via unofficial sites. If you download it from unofficial sites though, check that the zip is ~2.3 GB and that, if you open it with RAR or some zip manager, it has an update.zip inside and a PDF. If it has, follow the instructions; if not, just move it to the root of the sd card (/storage/emulated/0/) and go to Settings - Updates, and update. The A2017G has a list of previously unzipped system zips on the Guides section (Download Center by [MENTION=5334294]raystef66[/MENTION], check it out)

If you know how to use an IDM, it can be very good on the phone. Just sayin'

For the A2017: http://www.zte.com.cn/china/sitecor...rt/detail?id=B4C0ACE418834F588FED0C72D6046C78

For the A2017G:

http://www.ztedevice.com/support/detail?id=98CEB24F9FFD433EA99EC424163149A6

For the A2017U: 'Tis a little bit more fuzzy, there's many different carrier ROMs and there's only one zip from the unlocked model page. Either find a package here on XDA or, if you solely want to update to Nougat, go to the official page and download the zip available for your carrier:

https://www.zteusa.com/support_page/

Select 'Smart phones', the carrier, and ZTE Axon 7 (Not 'ZTE Axon' nor 'ZTE Axon 7 Mini'!)

So, process:

a. If you'll use the Phone:

Once you have the system zip, install RAR from the Play store or an apk, and find the Downloads folder, and the zip you downloaded. The screenshots here are just emulating how one of the official files will look, the numbers of the zip are just random and the sizes are not realistic (I just took some files and renamed them :p).

[IMG="1"]https://www.dropbox.com/s/l5ifcm6lqb8nvrb/Screenshot_2017-08-28-10-28-47.jpg?dl=1[/IMG]

Open the zip, find the update.zip inside, select it with the checkmark, and unzip it.

[IMG="2"]https://www.dropbox.com/s/pxvndaemzi95kl9/Screenshot_2017-08-28-10-29-01.jpg?dl=1[/IMG]

RAR will ask you where to extract this. Just remove everything after /0/, like on the screenshot. Press OK.

[IMG="3"]https://www.dropbox.com/s/kignycd8ii71n2g/Screenshot_2017-08-28-10-44-40.jpg?dl=1[/IMG]

Go to System Update on the Settings, and there should appear the zip, with an sd card icon. Press Update and you're done!

b. If you'll use the PC:

1. Download the zip on your PC.

2. Open your zip manager of choice and open the zip. Throw the update.zip on your desktop.

3. Connect your phone, open the internal storage and throw the update.zip inside.

4. Go to Settings, Updates, and the zip should pop up, with an sd-card thumbnail (even though it's not on an SD card, disregard that). Install, and you're done.