Need some help...

I just want to do something really simple, but on some certain way i can't get it done. I have a midifile wich contain some tracks. So i want to change some instruments in the Midifile (track 1 = strings, i want to make a piano from the strings). So i changed them and (re-)saved them with write button (write > song). But when i open the song again, the midifile still contains the wrong instruments wich i changed!

I think i missed something in the manual, and the solution may be very simple, but i can't get it done. I hope someone can help me out, becaus i get very frustrated -_-

Thank you in advance...