Programatically rewrite view fields in Drupal 8

Problem: I had the need to re-write some view fields programatically

Solution: 

1. first use template_preprocess_views_view_fields hook

   ex:- yourtheme_preprocess_views_view_fields(&$variables)

2. get the any field 

   $view = $variables['view'];
   $row = $variables['row'];
   // You can get the field value for the field this way even if the field is hidden (excluded from display)
   $field_value = $view->style_plugin->getField($row->index, 'your_field_machine_name');

3. Now assign a value to the field

   // To assign html markup
   $variables['fields']['your_field_machine_name']->content = \Drupal\Core\Render\Markup::create($html_string_here);

   //Raw value

   $variables['fields']['your_field_machine_name']->content = $raw_value

Hope this saves you some time