update python 3.9 ubuntu code example

Example 1: update ubuntu to python 3.85

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 1
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 2

Example 2: update ubuntu to python 3.85

sudo update-alternatives --config python3