When you tap on the level you wish to play and choose 'Start, this will take you to the level screen.
Before you make any moves, there should be two buttons that say 'UPGRADE FIENDS' and 'CHANGE FIENDS'.
Tapping on 'CHANGE FIENDS' will allow you to swap your team around. You'll also be able to choose the 'Auto-Select' Fiends which will select a team for you.
The option to have this auto-selection permanently on or off can be found in the game menu under Settings -> Advanced Settings -> Auto-Select Fiends.
Note: Once you make a move, the 'change Fiends' and 'upgrade Fiends' buttons disappear.