If set to default by MTU should be "
C:\Program Files\Micro Technology Unlimited\Microstudio\songs" If that isn't the folder you will have to search for the folder you set as the default folder by checking Tools/Default Folder in Microstudio.
Use windows explorer to find the folder the files are in. Find the file you want to work with, right click on it and click on "Rename". Highlight the last 3 characters (BIN) and type in"CDG". Simple as that.