Hi
If you want to totally remove the track you can right click it and select "Unlink from video" followed by a second right click on the track followed by "Delete"
If you feel you require the track to remain but not play you can mute it by right clicking it follwed by "Mute audio"
If you want bits of track 1 (or any track) to play then you can split the audio track in the selected places using the "Split" button under the preview screen and with Audio track 1 selected. (Use the down arrow to select the required track) You can then mute or delete the selected split off sections.
Audio on track 2 will mix with track 1 except where track 1 is muted or deleted.
Nat