title: Google Drive SDK: I want to update a file, not create new one
body: <p>In my opinion it should be very easy, but because of some reason I can't find the right way how to do it: how should I update a file in Google Drive using Python?</p>
  
  <p>My code:</p>
  
  <pre><code>from pydrive.auth import GoogleAuth
  from pydrive.drive import GoogleDrive
  gauth = GoogleAuth()
  gauth.LoadCredentialsFile("mycreds.txt")
  drive = GoogleDrive(gauth)
  file = drive.CreateFile({'title': 'Hello.txt'})
  file.SetContentString('test 1')
  file.Upload()
  </code></pre>
  
  <p>This creates a new file. Now I want to add to this file next line 'test 2'.
  Running above code each time creates new file, which is not what I want.</p>
  
  <p>Can anyone help me with this please?</p>
  
  <p>Daddy</p>