i put this in general for 2 reasons A. im not sure if this patching problem is limited to just all GOG games in general or just this one and B. its been 11 hours since someone posted in the arcanum forums :P

anyhow i just downloaded and installed arcanum from the downloader well i try patching the newest version of the unofficial patch and its states and error in opening the file for writing

can anyone please help me fix this please ^~^

Right-clic on the file that gives you that error, and untick the "read only" (or something similar) box.
Have you got windows and are you running as an administrator ?

What error is stated? If Narushima's advice doesn't work then try redownloading the patch. It might be corrupted.