You find it in the EXPORT_DESCR_UNIT.txt under the folder data for EB.

type barb brihentin gaul
dictionary barb_brihentin_gaul ; Brihentin
category cavalry
class heavy
voice_type Heavy_1
soldier celtic_cavalry_brihentin, 25, 0, 1
mount heavy horse
mount_effect elephant -4, camel -4, chariot +2
attributes sea_faring, hide_forest, hardy
formation 1.5, 4, 3, 6, 4, square, wedge
stat_health 1, 0
stat_pri 9, 55, no, 0, 0, melee, simple, piercing, spear, 160 ,0.33
stat_pri_attr ap
stat_sec 11, 35, no, 0, 0, melee, simple, piercing, sword, 0 ,0.22
stat_sec_attr no
stat_pri_armour 13, 7, 2, metal
stat_sec_armour 0, 0, flesh
stat_heat 5
stat_ground 0, -2, -4, 2
stat_mental 14, disciplined, highly_trained
stat_charge_dist 40
stat_fire_delay 0
stat_food 60, 300
stat_cost 2, 3647, 912, 60, 90, 3647
ownership gauls, scythia, britons, slave


Highlighted numbers are lethality. You need to edit each unit individually. I believe vanilla in it is set to 1 which is likely the maximum?