You will have to import a font file to your World Editor and then write something of a code in a notepad then also import it. Both imports must have no war3mapImported.. remove that in the path and the font file must end with .TTF or .ttf.
Now, if you have imported those two files correctly, the font will be changed but if it is wrongly imported be carefull, it will make your map crash always so I suggest making a backup incase this happens!