Disband
The disband page helps you control the order your army enters into battle. Using the fields to the right you can input how many you want to disband. To see how many you need to add or remove to change a stack position, hover your mouse over the + or - respectively. You have to check the "disband certification" box to disband. You will receive a message if you try to disband an "Undisbandable" Unit.
Picture of the +/- characters at work: