When you add a clip to the Timeline, the Video Track displays the video portion of the clip and the Audio Track displays the audio portion of the clip. In order to replace audio, you will have to delete the original audio first.
To speed up the process, you can zoom out the timeline to the extent where it is easy for you to drag the new audio and align it with the video clip.
You can also lock Audio Track 1, then right-click on the video file and choose to add it to Video 1 and Audio 2. If your MP3 file is on Audio 1, it won't be moved or affected when you add the audio to Audio 2, and you can then simply delete the unwanted audio.
