Changing music on the sound track

Hello, I have created a movie lasting for one hour twenty minutes to which I have added some music files. I wish to change one of the tracks already on the video for another of a similar time period. When I delete the piece of music from the video it moves all of the other music on the video to a different position, how can I resolve this, thank you! Even when I download the other piece of music the other files have been moved to a different place on the audio track/