Press "Edit Playlist" when a playlist is active to start editing.
While editing the songs in the playlist, you can sort and filter to find the songs you want to add to the playlist.
Press the red “-“ button to remove a song from the playlist.
Press the green "+" button to add a song to the playlist. The background color of the song will turn green or blue, and the song will also appear as the last song at the top of the playlist.
The songs already included in this playlist will be highlighted in green. The songs included in another playlist will be highlighted in blue.
To change the order of the songs, drag a song to the desired position using this symbol.
Press "Done" when you have finished editing.