Is there a way to manually update the gog version from 1.1.2 to 1.2.0 ??
or is it at least possible to take over the progress, as it seems i have to reinstall the whole game?
You just download the offline installer from your gog library and use that to install the game. I believe you can download the offline installer from gog galaxy as well. The latest version is 1.2.0e.