HOW DOES ENDER TURN THE BOTTOM BUNK BY THE DOOR FROM NEGATIVE INTO A POSITIVE? WHAT DOES THIS TELL ABOUT ENDER?

The other boys resent Ender for being Graf's "special" student so they leave him the bunk closest to the door. Ender, however, knew that battle school rules meant that front bunks are always left  for platoon leaders and the best soldiers. Ender uses the boys' insult against them.