Open the character folder and open the .cns file. Increase the number next to "attack =", which is underneath [Data].
If you want to change indivisual attack damage, go to an attack and change the damage.
Ex.
[State 200, 1]
type = HitDef
trigger1 = Time = 3
attr = S, NA
damage = 10, 0
can be changed into
[State 200, 1]
type = HitDef
trigger1 = Time = 3
attr = S, NA
damage = 20, 0
The number on the left is the basic hit damage and the number on the right is the shield damage.