You can move borders between slides to adjust slide duration.
To move a slide border:
Move the cursor on the timeline to the position where you want the presentation to switch to the next slide and then click Set Next Slide on the toolbar.
Duration of the other slides in your presentation won't be affected. However, if you want to move a given slide border without changing positions of the other borders, hold down the Shift button on your keyboard and then drag a border.
You can also move on-slide animations and transitions the same way.