Oké. So here is what I did :
To know where is my assets folder, I take a look in my \\BATOCERA\share\system.config\retroarch\retroarch.cfg config file.
There, I can see : assets_directory = “~/.config/retroarch/assets”
This means my assets netbios path is : \\BATOCERA\share\system.config\retroarch\assets
Then I downloaded the Batocera iso file from : https://updates.batocera.org/x86_64/stable/last/batocera-5.26-x86_64-20200527.img.gz
I extracted batocera-5.26-x86_64-20200527.img.gz archive on my hard drive with 7Zip.
Go to the extracted folder and extracted the vfat.img with 7Zip too.
In the \vfat\boot folder I found a file called batocera (1.13Go) and extracted it with 7Zip again.
It creates a folder named “batocera~”.
New Batocera 5.26’s assets are there : batocera~ \usr\share\libretro\assets
Finally to update my assets manually, I compared my folder content \\BATOCERA\share\system.config\retroarch\assets with the batocera~ \usr\share\libretro\assets folder content. And merge newer (add+replace) files to my assets folder.
I also did a merge with assets from the Retroarch dev repository at Github.
You can download it here : https://github.com/libretro/retroarch-assets/archive/master.zip