To remove the original audio and add a new audio to the video file, please follow below instructions
1. Click Import to load the video and audio files to the Media Library. Also drag and drop the video file onto the video track.
2. Select the video in the timeline, right-click and choose Audio Detach.
3. Then the audio part and the video part will be separated. Select the separated audio file and delete it.
4. Drag the new audio file from the Media Library to the audio track of the timeline. Meanwhile adjust the audio and move it to the place where you want it to be.