Dear Heike,
the expanding slider’s slides are ordered by their creation date (this is because if you choose the posts as source of content for the slider, they’ll be ordered in a logical manner). So you can simply change the “Published on” settings for each individual slide to re-arrange them: in the slide edit mode, refer to the “Publish” box on the top right; press “Edit” next to the “Published on” line, edit the date, and then press “OK” and “Update” below.
Sincerely,
Andrew | Satori Studio