sudo apt install --reinstall python3
sudo apt install --reinstall python
sudo update-alternatives --remove-all python
sudo update-alternatives --remove-all python3
sudo ln -sf /usr/bin/python2.7 /usr/bin/python
sudo ln -sf /usr/bin/python3.6 /usr/bin/python3
please note that some older Linux distributions were based upon python 2.x and you would corrupt the system if "python" was set to be a "python3" version. there were times where v2 was mandatory and v3 was optional. this meanwhile has sort of swapped.
beyond that i have not yet ever checked if alternatives selection will allow to have a per-system or per-user preference for a specific python major/minor version. instead there is lots of people going for per-project virtual environment tooling (venv, et.al.) in user space - or folks that use docker and similar runtime image setups - all is sort of a decoupling from the system's python and module setup.