How do you make Varnish cache update when Drupal content changes? I just started playing with Varnish cache. Looks like an awesome thing to have for speeding up page load times! 

How does one force the cached copy of a node/page/content to be updated when the content changes in Drupal? Would it be something in the VCL file? I am using the VCL from Four Kitchens.
There are many ways to achieve this. A very common way is by using the Expire module (https://www.drupal.org/project/expire).

It can easily be configure to clear specific pages, or sets of pages from varnish when they are changed. It has been well documented and is up to date, a great module and does exactly what you require.