You can change the duration of a task by resizing it. In order to do that, follow the steps below:

1. Move your mouse the end portion of the bar and you should notice a specific icon of the mouse.
2. Hold down the mouse and move the virtually displayed taskbar to the right if you want to increase the duration of the task, or to the left if you want to shorten the duration of your task.
3. Release the mouse when you are satisfied to the resulted duration of your task or length of its corresponding taskbar.

Also you can change the duration of the tasks by typing another value in the duration cell for the desired tasks.