Downgrade Safari Browser
From: Get the latest version of Safari for your Mac


  You can keep Safari up to date by keeping macOS up to date.
  
  Safari is included with your Mac. To get the latest version of Safari, install the latest macOS.
  
  To keep Safari up to date for an earlier macOS, install the latest
  updates for that macOS.


Note: Updated to reflect changes in Apple Support HT204416 as of: Jun 15, 2017