There are three ways to get to the dismiss screen: You either click on your avatar in the upper left corner and go to the "Army" screen. There you select one of the categories and then click on the unit you want to dismiss. Also, you can select the unit you want to dismiss from the Sanctuary in case they are wounded. Or you click on a military building, choose the option "View Units" and then choose the unit you would like to dismiss.

Once you are in the unit screen, a red trash bin is located in the right lower corner of the unit picture. When you click on that bin you are able to set the amount of units you want to dismiss. Once you have dismissed units, they are not retrievable.