How to change the value of a field in its preprocess method?
In Drupal 8 front page is a view called frontpage if the view is not modified it should show: "Content" by default (instead of fields).  If so the following can be applied to change each node title

function THEMENAME_preprocess_node(&amp;$variables) {
  //for views that show: "Content" (not fields)
  if($variables["is_front"]){    
    $variables["label"][0]["#context"]["value"] .= " end of title";
  }
}