If you decide that you don't like the sort order of the slides in your slide show and you don't want to exit the Slide Show Editor to go back to your Album and reorder them, or you didn't assemble your slide show from an Album, click Quick Reorder in the lower-left above the Timeline and drag to reorder as desired.
The Quick Reorder window is also a good place to get a quick count of the number of slides in your show for the purpose of, for example, making sure you have the same number of slides in your show as you have music beats on the PRE timeline.*
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*I find using the asterisk (*) to place the music beats myself--as RJ Johnston and Bobby, among others, suggested--to be preferable to using the Detect Beats Function in PRE7.