Try this:
1. While in the HDD mode, press the MENU hard key to access the HDD Menu.
2. Touch the My Files soft-key.
3. Select My Music
4. Select the My Playlists soft key.
5. Touch the EDIT soft key of the playlist you would like to edit.
6. Touch the Rename This Playlist soft key.
7. Type in a new name for the playlist.
8. Toucyh the Save soft key to confirm the name.
This is from the Navigation User's Manual.
Hope it helps.
Alan