assuming that you are talking about internal hdd storage mp3 player all you have to do is see if the hard disk is removable and connect it to a laptop with the usb cable ...or if it have a usb cable connect it to a computer and the hard disk will appear in the my computer area (folder ) by the device name ..or like a removable storage device ..(it will be better if it came with a driver for it ...a disk that will manage it )
but in most cases this is not needed ...eiter you connect it with the usb cable to a computer ..and in the folder I just describe you will find the music you mentioned...and just click on it ..and delete ..After that just drag the folders or copy and paste the music in the removable storage device in the my computer folder...
another way is look in the menu and choose the folders than on options delete them ..after that add the music by importing from a usb flash drive (stick ) or any other storage supported..
But the most simple way is if the hdd is removable ..and to place it into a computer ,,and add the stuff and remove the other you do not need..
if you provide me the full name and serial I will look for a manual but from what you described all I found is this one ..and it has no manual...http://www.techfresh.net/the-bose-new-car-stereo-system/
But from what they say ..it supports even transfer from an ipod or a usb stick ..so it may be conected to a coputer also ...by a usb cable ..
you can also copy the music from the disk by selecting the folders and copy to hdd..