1) Music can just be replaced if the name of the new file matches what you're replacing. The new one needs to be a .ogg file as well, but I'm not sure if it also needs to be a specific bitrate or something.
2) If you reach a point where you want to make a backup, copy the "saveData" and "Snapshot" files into a new folder. You can then drop them from the folder back into your "Data" folder and replace the files already inside. Make sure to keep a backup of your current save so that you won't have to redo stuff.