[Paul Riley]

Simply browse to Ubisoft\Silent Hunter 3 and look for a file that is called .cfg and open it.Then look for an entry that alters resolution,as maillemaker mentioned this entry should be obvious as the current resolution will be displayed next to it,then edit the file to your desired resolution,then at the top of the screen click 'file' then 'save' then 'save as' ,then overwrite it,and then close the edit screen.
This then should enable the resolution you want.

NOTE: always have a backup of your original .cfg file,just in case.And do this with any other file you decide to edit in the future.