How can I delete tracks from my Goodmans MP3 player?

I have a Goodmans GMP317 MP3 player and would like to delete some tracks from it but when I attach it to my computer it does not show all the tracks which are currently on the MP3. It just shows the music I have downloaded using that particular computer. I have loaded the music using different computers but surely I should be able to see everything that is on the MP3 as it is an external drive. I can't work out what to do and neither can my daughter. I am sure it is something simple but I need someone to help me please!