Midi output deviceproblems

Hello
OK
But when I"ll want to re-open the save.not version of the file I don't have the change that I saved.
I believe that come because during my work a window appeared"reassign staves to new default device" and that I do something wrong in this window.
I think that I made an error by selecting the change for all the songs and not for this only song. After several handling I succeeded in opening this window"reassign staves to new default device "and now all turns correctly, but how to reach this window more easily