just open your copy of iTunes and find that playlist.
Arrange the tracks in the order that you want.
Attach your iPod and Sync.

The tracks on the iPod should now be in the same order as you want them.

Alternatively, you can attach the iPod, remove that playlist from the list of playlist that the iPod syncs with and that will remove the playlist from the iPod.
In iTunes, rearrange the tracks in the way you want them
Attach the iPod and set the iPod to sync with that playlist again.
OR
In iTunes, select the offending playlist, and select Edit > Delete
Choose File > New Playlist and start again from scratch

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