Make sure the slides and animations are set to trigger on click so you can see them properly on the timeline.
To change the duration of the slides and transitions, drag the marker between them on the timeline.
You also can move the cursor to the position where you want the presentation to switch to the next slide. Then click Set Next Slide on the toolbar. The duration of the other slides in your presentation won't be affected.
However, if you want to move a given slide marker without changing the positions of the other slides, hold down the Shift button on your keyboard and then drag the border.
You can also move on-slide animations and transitions the same way.