I want to post but some error give from wpcentar (maybe because i want to post links). I try to PM to you but you disable that. All process is very easy, but i order new battery. This method is only for international version of Ativ S GT-I8750
http://forum.xda-developers.com/showthread.php?t=2391138
on 22 page you will found a working links
step by step instruction:
1. Download Flashtool+Driver and ROM with CSC file.
2. Install Driver on your PC (i had problems with win 8.1 X64, win7 X64 installation fine)
3. Put your phone in Download modus
Shut down your phone
press and hold volume up + camera button and press shortly power button (still hold volume up and camera buttons!)
download modus must appear.
4. Start flashtool and load Rom (.WP8 file) and csc file into the flashtool
in the yellow folder load Rom. (.WP8 file)
in the green folder load CSC file
5. Choose your default CSC Code under DL Options-->CSC sale Codes. (for example DBT is Germany)
6. Plug in your phone to your PC over USB cabel.
7. Press start in the flashtool (if you cant press start, something wrong with drivers or phone--PC connection)
Flashtime about 10 min....
I am NOT responsible for any damage to your device!
You do everything on your own risk!