depends on item you want to change the icon..
if it's weapon you will change the icon name on weapongrp.dat file if it's armor on armorgrp.dat file if it's item like adena for example you will change it on etcgrp.dat file.
you will find in line of the item you want to change the icon a name like this icon.IconName and you will change it to YourTextureName.YourIconName