1. Create your video and open it.
2. Click on the 3 dots and Open in Composer.

3. Choose the language of the captions.
4. In Composer you will have a storyboard with your captions, you can make changes on the segments by clicking on the 3 dots:

You can merge your segment with the previous or next one.
Split segment in two - a segment editor will pop up. Divide your captions into two scenes.
You need to copy the second part of the text after splitting it.
You can split more than two times by clicking again on the 3 dots and click on split segment into two. You can do it as many times as you need.
Edit segment duration - a segment editor will pop up.
Drag the blue bar to change the length of the segments.
Click on the arrows to move between the different scenes.
When the button Done! appears, it means that you have finished. Now Click on Save.

5. Click on Save and Create to apply the changes.