You edit a non-smart playlist by merely deleting songs from it; dragging songs to it (drag a song to the name of the playlist in the sidebar and drop it), or you can go to the songs list, click the three dot icon next to the song, and click "add to playlist"; manually dragging to change the play order (or sorting by one of the columns); etc.
If it is a smart playlist, the edit allows you to change the rules on how media is selected for the playlist.