title: Update existing virtualenv to use Python 3.6
body: <p>I have an existing virtualenv called 'edge'. It uses Python 3.5.2. I have upgrade my Python interpreter to 3.6 and I want the 'edge` env to use 3.6 instead.</p>
  
  <p>What command should I used to <code>update</code> edge's interpreter?</p>
  
  <p>I searched on SO but all the answers I can find are for creating a new env. In my case, I don't want to create a new env.</p>