I want to change an mp3 track to 18000 hz and make inaudible to humans how can I do that
You can change the sample rate with the little window at the lower left of the Audacity program. But that won’t make the show inaudible. If you do manage to shift musical tones down so far that nobody can hear them, you can’t come back. This is a cousin to the MP3 dilemma. Once you compress the life out of an MP3 file for small file size, you can’t ever make the quality any better than that no matter what you do later.
So what’s the real job?
This topic may be relevant: Silent Subliminals [solved?]