I'm using swift code but is not changed file extension.
This my swift code
let url = NSURL(fileURLWithPath: outputURL.path).deletingPathExtension?.appendingPathExtension("mp3")
You are just creating a URL and then changing that URL. You are not actually operating on any file.
To rename a file, call FileManager.default.moveItem(at:to:). Pass the file URL as the first argument, and the file URL with the new name as the second argument.
let newURL = outputURL.deletingPathExtension().appendingPathExtension("mp3")
try FileManager.default.moveItem(at: outputURL, to: newURL)