Hi Jean
One method is to go to step record for the tracks you want to change sound in and cursor along until you find an asterisk that shows the GM sound. Delete it and insert your chosen sound. If you play the file and go to Mixer, you will see the sounds for each track. On some GM files the sound change can be several bars in - so make sure your chosen sound is input just before the first note.
No doubt there are other ways of doing it, but this should help till a better suggestion comes along
John