In the View All Phases Dialog you can use the check boxes on the left hand side to select multiple phases or tasks and then use 'Add Weeks' Or 'Shift Move' buttons to move the selected phases.
The Shift Move option will move the selected phases or task start and dates the entered number of weeks.
The Add Weeks option will add the number of week entered to the end of the selected phases or tasks. However, any planned staff or assignments will be given 0 hours for the additional weeks, this allows a manager to extend a schedule but only plan select staff manually.