How to change Date format from YYYY-MM-DD to DD/MMM/YYYY

  mutate { add_field => { "[date1]" => "2018-05-19" } }
  ruby { code => "event.set('date2', Date.strptime(event.get('[date1]'), '%Y-%m-%d').strftime('%d/%b/%Y'))" }

does it. Error handling left as an exercise for the reader.