export_descr_unit.txt

make a back up copy of the file first and change the value after stat_mental :

type templar militia
dictionary templar_militia ; templar militia ; spear - sword
category infantry
class spearmen
voice_type Heavy_1
soldier templar_militia, 40, 0, 1.1
;officer standard_albion
attributes sea_faring, hide_forest, can_sap
formation 1, 1, 2, 2, 5, square, phalanx
stat_health 1, 0
stat_pri 8, 3, no, 0, 0, melee, blade, piercing, spear, 25 ,0.6
stat_pri_attr spear, long_pike, spear_bonus_12
stat_sec 10, 4, no, 0, 0, melee, simple, piercing, sword, 25 ,0.9
stat_sec_attr no
stat_pri_armour 9, 4, 6, metal
stat_sec_armour 0, 0, flesh
stat_heat 2
stat_ground 1, -1, -3, -1
stat_mental 9, impetuous, trained
stat_charge_dist 3
stat_fire_delay 5000
stat_food 60, 300
stat_cost 1, 500, 150, 40, 60, 500
ownership empire_west
The one marked in red - higher is better morale and less likely to rout. There is a good 'complete guide to ... ' that file in the scriptorium now which might help you with some of the other aspects.