How to change the duration of a scene?
On the scenes or layers timeline, you can drag the scene handle to the right to increase its duration, or to the left to decrease its duration.
You can also right-click on a scene and set its duration manually.
Changing the scene duration will also change the duration of all the elements in it proportionately.
To change the scene duration without affecting element durations in it - in the layers timeline, you can increase the duration of a single element to expand the scene, or, decrease the duration of the element closest to the scene end to decrease scene duration. A copied existing element can also be used to achieve this.