Lower the exp required per level. In turn, exp gained from creeps need to be scaled down as well.
instead of changing XP from creeps, we can set hero XP tax to be 50% or whatever rate we choose.
I checked game constants and they are like 8 lines related to XP...
Don't know which ones I have to change
well, first of all, you can specify XP required for each level as you like in a table (last option). if you leave the table empty, then the formula will be used.
There is a formula which goes like:
XP required for next level = previous Level XP (which you already gained) + Level * Level Factor + Constant Factor.
E.g. level 1 requires 200 XP, and lets say level factor is 100
to go to level 3, XP = 200 + 3*100 = 500
to go to level 4, XP = 500 + 4*100 = 900 xp require etc.
Now if you used constant factor of 500 and XP factor of 50,
XP required for level 2 = 0 + 50*2+500 = 600
XP required for level 3 = 600 + 50*3+500 = 1250
XP required for level 4 = 1250 + 500 + 50*4 = 1950
and so on.