I don't know of any way to rearrange them as you'd like to. If you remove them from the stack as Matty suggested and then drop them back in it'll usually put them right at the front, but that's an awkward and time consuming way to go about it, and then it'll probably auto-rearrange them for you by the next turn or so anyway. Army organization in general is a little quirky and frustrating, but the game has plenty more good qualities to balance it out.

Ajax