The Edit Page : Editing Your Project : Transitions

Transitions
To add a Transition, touch and hold the item and then drag it to the desired location on the Storyboard. If you drag a transition over another transition, it will be replaced with the new transition.
Transition Length
To adjust the duration of the transition, tap on the transition to display the trim handles in the timeline, touch and hold a trim handle, then drag to adjust the duration of the transition.