You can manually create a song from admin area -> albums page by clicking pencil icon next to an album and then clicking new track button.

 

Existing tracks can be updated from admin area -> tracks page by clicking pencil icon next to a track.

Note:

New Tack button will be disabled when creating an album. You will need to first create album and then edit it in order to add tracks.

Both of these will open track modal, where you can fill in track information, url to track, as well as upload custom music file by clicking on upload button.