You need to move everything in the /forum dir to the root directory. You also need to change within admincp of forum location to / only
That is not the only thing. The forum url has to be updated as well to point to the new location at Settings->Options->Site Name / URL / Contact Details->Forum URL.
First change the url in the admincp (as noted in post 3) and then move the contents of the /forums directory into your root. You may need to adjust stylevar and attachment/avatar/signature/post icon/smilie paths depending on how you entered the paths in the first place.