If you use Notepad++ (or Sublime Text) open the spawn.xml file which belongs to your map.
Run Search & Replace (Or Find & Replace or something like that) and enable REGULAR EXPRESSION!
In search field write (replace ONLY monster name you wish to search for but don't remove the parantheses or anything else):
Code:
(name="Ice Golem" x="\d+" y="\d+" z="\d+") spawntime="\d+"
In replace field write (same here replace ONLY spawntime to the new number you want):