If you want do it by phone, then I think that you have to recreate a new one ...
If you use your laptop or computer, then you can edit any of your playlists ... connect your cp by USB cable to your computer, search pla file from your music folder or playlist folder, and right click it by your mouse, chose properties and then middle tab there - I think that it is referencies (sorry, but I do not remember it, as my OS is not english)

... then you can add, remove, change order songs in your playlist .... please do not try to move your playlist (it does not work), just edit it ...