how to change the quality of a downloaded video pytube code example

Example 1: pytube thumbnail

YouTube('https://youtu.be/9bZkp7q19f0').thumbnail_url

Example 2: download video to from pytube with a special name

import os
from pytube import YouTube

yt = YouTube('http://youtube.com/watch?v=9bZkp7q19f0')
yt.streams.first().download()
os.rename(yt.streams.first().default_filename, 'new_filename.ext')`