I am french, so if the staft wants to create a french category, I can post this publication in french 
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I’m publishing what I’ve done. This method will surely work on other models.
The only procedure shared by users concerned the TS-x19x model (TS-219, TS-219P, etc.). It consisted of downgrading the Fujitsu firmware, before installing an older version of the QNAP firmware, and finally updating the latter to the latest version.
I installed the latest firmware directly and it worked.
Save all your data,
This method uses a manual firmware installation procedure provided by QNAP.
I’ve included links to the resources at the end.
You need to open a console (terminal) capable of using SSH.
On Mac and Linux, this capability is usually enabled by default.
On Windows, you either need to enable the “OpenSSH Client” feature in the settings or install “Git Bash” or “PuTTY”.
1- Download and extract the Qnap firmware you want to install. You can rename it. I named mine “TS-221.img”.
2- Copy the Qnap firmware (TS-221) to the Public Folder of your Q703.
3- In a SSH console (terminal) type the following commands. You can copy and past them. Don’t forget to change the values for your case (ip of the nas, name of the image).
ssh
ssh admin@ip
Replace “ip” with the ip of the nas. On Linux or Mac, the command is ip@admin.
Then asks you for password by default is “admin”. Then type:
mkdir /mnt/HDA_ROOT/update
rm -rf /mnt/update
rm -f /tmp/update_process
ln -sf /mnt/HDA_ROOT/update /mnt/update
/etc/init.d/update.sh /share/Public/TS-221.img
Then the update script runs, you will get some output feedback in the terminal.
The flashing process takes some time.
If everything goes well, the procedure ends with the message: “Update Finished”.
Then type:
reboot
Arfer reboot, You will find the Qnap QTS interface on the NAS webpage!
In my case, everything was preserved as before; all my applications and data were still there.
The NAS seemed to be working fine, as if I had simply performed an update. So I hadn’t reset it.
But after a few hours of use, I started getting error messages! I then tried a reset without formatting the drives.
But the system forced me to perform a complete reinstallation with formatting. It also requires reinstalling the firmware. It simply asks you to select the image file (TS-221.img in my case).
It’s therefore best to perform a complete reset after changing the firmware. So be sure to back up your data beforehand!
You have to follow the procedure, and you end up with a QNAP that’s like new.
It’s possible to create a QNAP account, register the NAS on the QNAP website, and use all QNAP services.
I suppose this method works with all NAS devices running QTS, as the documentation doesn’t specify any particular NAS models. Please share your experiences or post them on the forum.
Best regards. Zikko71
I used the following sources of information:
https://www.qnap.com/en/how-to/faq/article/how-to-manually-update-the-firmware-by-ssh
On The following line is incorrect. It is useless:
cp /share/Public/TVS-X63_20151118-4.2.0.img /share/Public/TVS-X63_20151118-4.2.0-work.img