[Sony Xperia 1 II Guides, News, & Discussion] brocnical:
You can find out your current customization by dialling *#*#SERVICE#*#* -> Service info -> Software Info -> Customization Version. This guide explains how to flash another official Sony firmware of different customization onto your Xperia 1 II XQ-AT51. You do NOT need to unlock your bootloader to do this.

Download and install FlashTool from http://www.flashtool.net

On Windows you will then need to install the Flashmode and Fastboot drivers.

On Linux you do not need to install drivers, but you do need to set some udev rules.

On Windows you will first need to reboot Windows with driver signature enforcement disabled to do this. Then install the Flashmode and Fastboot drivers from the installed C:\FlashTool\drivers\Flashtool-drivers.exe:

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Open Flashtool and let the device database sync

This may fail on Windows if you have Windows Defender Antivirus Real-time protection running -- disable Windows Defender Antivirus Real-time protection in Windows settings:

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Click on the XperiFirm icon to get a list of official Sony firmwares:

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Select the XQ-AT51 on the left, and the select the particular Customization you would like. Then click on the box below "Available Firmware" to start downloading:

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Once the Firmware has downloaded, close XperiFirm and then let FlashTool do some processing on the downloaded firmware to create a .ftf file of the firmware in your C:\Users\<username>\.flashTool\firmwares folder.

This step is no longer needed as the FlashTool database has now been updated. This step is temporary and will not be required once the FlashTool devices database has been updated for the XQ-AT51.

Download the .fsc file and .properties file and unzip them into a new folder in C:\Users\<username>\.flashTool\mydevices\XQ_AT51 . Rename the .fsc file to "58.0.fsc" to use it with more firmware versions. Then in FlashTool's menu choose Devices->Manage->Import -- this will give an error, but ignore.

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Now click on the Flash button and choose Flashmode.

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Set up the Firmware Selector window as follows - this setup will erase all data on your phone. Then click Flash:

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When prompted, power off your phone and then hold down the volume down key on your phone and connect a USB cable from the phone to your computer (do NOT use the UCB24 USB C - USB C cable that comes with the phone -- the UCB24 is for charging only and does not work -- you need to use a quality USB C - USB A cable or try the UCB32 instead). Keep the volume down key held until the notification light turns green. The flashing process will now be prepared.

When you get a prompt saying "A FSC script is found. Do you want to use it?" choose yes. An error of "those data are not in the fsc script: persist, reset-non-secure-adb" is entirely normal, so choose Yes, and let the flashing commence. If you get a prompt asking you to select a partition, select the top option for 128GB storage and the bottom for 256GB storage -- if unsure click cancel. If you get "ERROR:null" then repair your phone with Xperia Companion first.

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Flashing will be complete when you see "Sync status : OKAY".

If you are worried as to whether the flashing worked or not, you can always then do a software repair using the Xperia Companion software. Please check your storage after flashing - some users are reporting that they only are seeing 128 GB of storage instead of 256 GB - please repair with Xperia Companion if you see this and report in the comments below.