title: Remove old version of python and replace it
body: <p>I am using Ubuntu 16.04 LTS. I installed python 3.6 using <code>sudo apt-get python 3.6</code>. But python 3.5 is still installed. How do I remove 3.5 and set 3.6 as default in place of it so that when I type <code>python3 --version</code> in terminal it shows 3.6.5 instead of 3.5.2</p>