You can try changing the unit's level, that should alter EXP given.
In game play constants, there are a lot of fields to do with EXP, look at them to see if any alter EXP in the way you want.
You can also make your own EXP system using triggers which clearly enables you to set the EXP given or taken from units but I recomend this only if the other 2 do not fit your map and you know JASS.