you can only change armour of your mercenary soldiers that you can buy at any of the 5 mercenery camps on the map.
but you have to be careful because it could get very expensive.
and if you need money, you can attack enemy villages or sale stuff that you can take from battles. and it is really important that you have to look that you have not too many soldiers, because some of them are really expensive.
but after a while, when you play te story line for an empire, you can get castles and villages, and weekly taxes from your fiefs.