how can i convert protected mp4 file to mp3

Hi, can someone please explain to me how i can convert protected mp4 music files to mp3, because I want to transfer them to a usb to listen to them in the car. I have tried clicking convert through the edit tab, but i keep getting a message saying "itunes protected files cannot be converted". Please help, because it is doing my head in, i have tried all sorts of different ways and I cant do it.