Before we begin, you need to install ffmpeg, if you haven't already. You can install ffmpeg as described in this article.
Step 1: Go to Windows Start menu, search for "cmd" and run it.
Step 2: Navigate to directory where your video and audio files are located.
cd "C:\path\to\your\directory"
If the files are not in a C drive, you must go to the drive you will be working on with the following command before using the above command:
D:
Where D is the drive you want to go to.
Step 3: Run following command:
ffmpeg -an -i "video.file" -vn -i "audio.file" -map 0:v -map 1:a -c:v copy "output.file"
Where "video.file" is your video file, "audio.file" is the new audio you want to use and "output.file" is you resulting file.
If you dont want to re-encode audio stream you can use "-c copy" instead of "-c:v copy", but this may not work in all cases.
Now we are just waiting for the operation to end and we are ready.