In addition to uploading and creating your own songs, you can also edit existing songs that are uploaded by other users. With every song in the songs pane, you will see some options:
If the song was originally created by you, you will see "edit" and "delete" buttons in the contextual menu. Clicking "edit" will take you to the editor to edit that song.
If the song was originally created by another user, you will see a "create copy" button. Clicking it will take you to the editor to edit that song and save a copy of it. (You will not be affecting the original file.)
If you edit songs made by other users by creating a copy, you will be prompted to provide a new name for the song in the save screen.