update python in ubuntu code example

Example 1: ubuntu install python

# first do:
sudo apt-get update
# after do:
sudo apt-get install python3.6
# then it should be installed

Example 2: update python ubuntu

sudo apt-get update && sudo apt-get upgrade
sudo apt-get install python3.7

Example 3: update python ubuntu

$ sudo apt update -y
$ sudo apt install python3.7