upgrade python 3 to python 3.9 ubuntu code example

Example 1: update ubuntu to python 3.85

sudo apt-get install python3.8

Example 2: how to upgrade python from 2.7 to 2.9 on ubuntu 14.04

git clone https://github.com/pyenv/pyenv.git ~/.pyenv
 git clone https://github.com/pyenv/pyenv-virtualenv.git ~/.pyenv/plugins/pyenv-virtualenv