Under Advanced - Gameplay Constants, find the field Hero XP Gained - Creep Reduction Table. Its value will be 80,70,60,50,0. That is a percentage of the actual experience that the units give. Set the first value to 200 and delete the other values; all heroes will gain double experience for kills. It will also allow heroes past level 5 to gain experience from killing creeps.
If you mean the actual amount of experience to level up, I haven't worked out which fields those are. I'm sure somebody else knows, though.