In order to make a change to a preview, you will need to
1. click the "customise" button
2. Click the "Edit style or clips" button
3. Click the "Create New Version button
4. Select "Choose Clips"
5. Click the "Edit my clips" button
At the bottom of the screen you will see a minimised timeline, if you expand it you will see all of your chosen clips.... they can be dragged and dropped into place.
Pressing the X button at the bottom right of each clip will delete it from the timeline
(Open image in new tab to see the process in fullscreen)