The final Raid set-up screen should look something like this:
From this screen you can change which row a Raider is standing in. Melee characters can only attack from the front row. Healing and Ranged attacks can be performed from either row. Raiders in the back row take slightly less damage from melee attacks and draw less attention from monsters than those in the front row.
To switch a Raider's row, simply click on the current row symbol and you will see them move to the opposite location. Using the keyboard, you can press the number key for that Raider (or use the up and down arrow keys to traverse the list). Once a raider is highlighted, you can press the Spacebar to change their row. When you are happy with row assignments click the Confirm button or press the Enter key. The Back key will return you to the previous Raid set-up screen, typically DPS Assignment.