$ python --version
Python 2.7.13
# update-alternatives --list python
$ ls /usr/bin/python*
# update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
и
# update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2
# python --version
Python 3.5.3
# update-alternatives --config python
https://unixhow.com/3557/kak-izmenit-versiyu-python-po-umolchaniyu-v-debian