First, let's remove the master audio track that's already on the video file. Right-click the video file, and it'll highlight with a drop down menu like the one you see above. Choose "Detach Audio", and you should see the audio file become a separate entity on the editing line. This will be purple, showing that it's no longer part of the video file's integrated contents.
Now that you have your audio file separated, you're able to easily go in and edit this file. Clicking the small selector box on the left hand corner, you're able to make various EQ and fade adjustments to the original audio file; if you wanted, you could keep this audio file and simply mix the new one over the top; if you're going to fully replace the file, now's where you can delete the file entirely.
Now that you've moved your old audio out of the way, it's time to add your new audio.