GOG Galaxy - My Games Library - game page - additional - downlaod offline backups - download patch.
Currently you can download this patches (last one update for this game is few month old for now):
http://www.gogdb.org/product/1423049311#downloads Alternatively, you can do this using normal web browser - gog.com website - account - my games library - game page - download offline backup installers - download patches.
I cant guarantee it instalable on GOG Galaxy build. You probably be able to extract it with UniExtract2:
http://github.com/Bioruebe/UniExtract2 Andprobably this is even better than normal install.
Its strange that you are 12(or more?) years user of GOG and you ask this.
I dont know why, but i should warn you also about you need download not only exe file, but all bin parts of patch - they labeled as "part x of x".
last patches usually not available. It rare thing on GOG. Offline installer for this game been updated last time without verson change and there is no separate patch update file for it:
http://www.gogdb.org/product/1423049311#changelog If you want get latest offline build complete installer - you need redownload and reinstall whole package.
But this likely some minor installer update - no game files are changed.
If you update GOG Galaxy build using offline pathes (UniExtract2) - do VERIFY process in Galaxy client. At least twice in a row (since clint wont tell you result of first verify).
Whatever, not matter how you do that - each dowanload\update process should be verified. At least twice in a row.
