How can I unapply a subdivision surface modifier? I accidentally applied a subdivision surface modifier, but I don't want that. I tried to undo it, but that is not working. Is there any other way to do this?
You can discover the global history. If you can find the operat，then you can back to there by click.

following the link：
http://www.blender.org/manual/getting_started/basics/undo_and_redo.html?highlight=history