[Xperia Z4/Z3+ Q&A, Help & Troubleshooting] Avi_Sahani:
After hearing a lot of people having problem with the Sony Official Nougat update (7.0 ver. 32.3.A.0.376) in past recent months, I hence decided to make a guide on how to downgrade (6.0.1 ver. 32.2.A.5.11) for N00bs​

/*
* Your warranty will not be void.
*
* I am not responsible for bricked devices, dead SD cards,
* thermonuclear war, or you getting fired because the alarm app failed.
* Follow instruction properly and you are good.
*/

Prerequisite

1. Xperia Z3+ drivers (adb, fastboot/s1 drivers)

2. Flashtool (optional Xperifirm, it comes with XperiFirm in built) Dev by [MENTION=2807379]Androxyde[/MENTION] -

FlashTool by Androxyde

3. Marshmallow Rom (in .ftf format)

4. Phone with atleast 70~80% battery

5. Windows 7 is higly recommended so you can install unsigned drivers for your phone without any hassle

WIndows 8 and 8.1 - Flashtool/Fastboot ADB Driver Fix​

Notes:

Make sure to Backup all your files (Internal Memoy) and keep it somewhere safe on PC or External Storage, as this method will erase everything from Phone.

This method is working on Locked Bootloader and Non Rooted device, so it is advisable to do it in the same conditions otherwise official roms won't work.

-: SO LETS BEGIN :-​

1. Install Right Drivers for your phone

Flashtool and Fastboot Drivers Pack - Only Check FlashTool Drivers and Fastboot Drivers no other divers required while installing.

S1 Boot Driver - Download and Install it via Device Manager How to Install Driver for Xperia from Device Manager
​

2. Open Xperifirm, find your device on left window

In flashtool click 'XF' button on utility bar​

3. Find your Market (for example, India) and your Operator (for example, Customized IN)

4. In XperiFirm far right under your Xperia Model Preview , you can see available firmware - Download the one which you see is Marshmallow, Click Download and wait till it finishes.

(for example - 32.2.A.0.305 and 32.2.A.5.11 are same Android Marshmallow variant with different security patches and bug fixes but for different countries, UK and India respectively, so it will work just make sure it is Marshmallow by Googling the version number)

5. After firmware download completed, open FlashTool -> Tools -> Bundles -> FILESET Decrypt. And now locate your downloaded firmware files which is 'FILE_XXXXXXXX' and other many more like this in a folder, select the containing folder. Now in FlashTool window select all the files with same name structure (example: FILE_58854456) if there are any other files like 'fwinfo' or 'info' DO NOT SELECT IT, I RECOMMEND DELETE IT FROM THE FOLDER. After making selection hit the " -> " key , it will move the contents to right. Click Convert.FILESET Decrypt Tutorial with Pictures (External Site/NON-XDA)

6. After 'Decrypt' completes it will open a 'Bundler' window, here you need to select file from left and click ' -> ' to move it to the right, you need all files except xml files (example: boot_delivery.xml, fota.xml). The TA files didnt made any problems while flashing or booting in my experience. So, Include '.sin' and '.ta' files and no problem will prevail.

7. In FlashTool now, plug your phone (do not use MTP mode if asked on phone, keep in Charging Only) and you will see a message on FlashTool [ Device connected with USB debugging off ] so go to Settings -> About -> Build Number (Tap 5 Times) -> Settings -> Developer Option -> Enable USB Debugging. [Phone if asks - Allow USB Debugging on this device? - Click Always allow and then Yes/Okay]

8. In FlashTool click on Thunder icon from Utility bar, and select Flashmode. After this Open your rom folder were you downloaded (.ftf) earlier.

9. Choose your Device filter name by clicking the greyed out panel. Write Branding (= Operator from XperiFirm) and Version (in my case 32.2.A.5.11) of your Rom. Now Select eveything from '"Wipe Box" - Cache, Data, APPSLOG .etc' (will clear everything from your device, make sure to backup your internal storage)

10. Do not check or Uncheck anything from Exclude Sin and Misc. TA box. Click on flash.

11. FlashTool will show a mesaage Preparing files for Flashing wait for it, after that it will tell you to go into fastboot mode, to do this simply follow this:

-> Disconnect USB

-> Power off Device

-> Press Volume Up button Phone

-> While Pressing Volume Up Button Connect USB Cable

Now a Green and Blue led will blink and stays constant blue/green there while Fastboot starts flashing files.

12. Now after the flashing completes the device will reboot and you will get Marshmallow. :fingers-crossed:

Don't forget to say Thanks if it worked for you, It is my first time posting here on XDA Hope you liked the tutorial.

Ask questions i will try to give answers. :)