steps are one way to delete unwanted audio from a home movie
and then substitute it with a piece of music.
Step 1. load the movie file to the video track of the timeline.
Step 2. copy the video clip (Ctrl+C) and paste (Ctrl+V) it
to the first audio track.
Step 3. mute the video clip on the video track.
Step 4. find the audio segment to be deleted (or replaced)
using the MarkIn and MarkOut markers.
Step 5. split the marked segment using the "Split (Ctrl+Q)" function.
Step 6. mute (or fade, or sound line edit) the separated audio segment.
Step 7. add your music clip to the 2nd audio track and align it properly.
The result is shown here.