ubuntu set default python to 3.9 code example

Example 1: set python 3 as default ubuntu

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.4 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.6 2

Example 2: set python 3 as default ubuntu

update-alternatives: error: no alternatives for python3