I am trying to take the audio off of one video clip to use on another. When I tried to download it I was directed to a help page which suggested that i download the FFmpeg library. I did this, but I can not figure out how to run it. It is not under my ‘start/programs’ bar and I don’t see a link anywhere in Audacity to the FFmpeg library. I know it is on my computer only because I can find it under 'computer/C drive/Program files, But can’t open it from there, I can only view the files. How do I open and use this library? 
I moved this from “Adding Features to Audacity” because FFmpeg is already a feature and I think you are asking for help on Windows.
You have to go Edit > Preferences, choose “Libraries” on the left, then (if Audacity has not already detected FFmpeg), choose “Locate…” then “Browse…”. This is described in the Manual Audacity Manual .
You can run FFmpeg on its own from Program Files, but it is a command-line program so you have to open a command prompt at the FFmpeg location then know all the correct commands to use.
It is easier to open the video file in Audacity then Audacity will extract the audio from it and you can export it as an audio file.
If you export to exactly the same audio format you “may” be able to replace the old audio with the new audio without re-encoding the video, for example with avidemux. If you have to re-encode the video you will lose some video quality.
Gale