how can i change weapons on units

[Spoofa]

I find secondary weapons better for horsemen in a melee fight, i use my Equites all the time for flanking and they seem to do beter with their swords then the spears when it comes to death rate of the enemy, even in horse vs horse battles the sword seems to do a bit better.

Edit: i was looking at the hetairoi card, and the spear has less attack(4) but higher lethality (.4) and the sword has higher attack(11) but lesser lethality (.11) so im not sure what this means, nor do i understand why the sword is more lethal enless it is only meant for the charge.