Songs can be sent to the watch through the chat of the Xplora app.
In the chat of the new app, click on the (+) icon and attach an mp3 file (up to 5MB).
The watch will receive the song in the chat and, once it's played, you'll see the option to download it. The song will then appear on the Music screen of the watch, so that you can make a playlist.
If you want to delete a song, go to the Music folder and swipe the chosen song to the left. The option to delete a file will show up next to the song.