You switch Heroes in and out of your team when viewing the Hero Roster by tapping Edit Team. Tap the name of the Team (underneath the ‘Edit Team’ text) to change the team you wish to edit. Once you have selected the Team you wish to edit, you can change the Heroes assigned to this team by tapping on the Hero and selecting the red ‘x’ to remove the Hero.

To place a new Hero into the team, simply tap on the empty slot and tap the Hero you wish to choose from your Hero Roster. Once you have the Team fully edited, simply tap ‘Done’ and lastly, 'Apply'.