Edit or modify a task
While in the Gantt View, click on the task you would like to edit and then got to the detail window below and click on "update this task"
To change the start date of a task while in the Gantt View, click on the task bar and while holding down drag it over to the new start date. Notice all dependant task will move accordingly. If you want to move just that one instance, hold down the shift key and then drag the task bar to the new start date. Notice only that one task moved. An example of why you would want to do this is as follows: Currently, you have the plumbing contractor on a job and the heating contractor is going to be 1 day late. Knowing the two sub-contractors can work together you can move the start date for the Heating contractor over 1 day which will allow you to maintain your scheduled completion date.
To change the duration of the task while in the Gantt View, click on the left edge or right edge of the task bar and drag it to the new date or in the detail window below on the task information tab, change the duration or the days delay and then click on "update this task".