upgrade python3.9 ubuntu code example

Example 1: update ubuntu to python 3.85

sudo apt-get install python3.8

Example 2: change python 3.5 to 3.6 ubuntu

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.5 1

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


sudo update-alternatives --config python3