How to modify an existing view join?
Usually, you should control join type using views Advance relationship options. By enabling "Require this relationship" you are telling Views to use "INNER" Join instead of using "LEFT" Join. If this option is somehow not working here is how I would solve this problem.

use \Drupal\views\ViewExecutable;
use \Drupal\views\Plugin\views\query\QueryPluginBase;

function hook_views_query_alter(ViewExecutable $view, QueryPluginBase $query) {
   // check your view ID:
if ($view-&gt;id() === 'id') {
 //Get your table info object;
  $table = $query-&gt;getTableInfo('table');
if (isset($table['join']-&gt;type) &amp;&amp; $table['join']-&gt;type === "INNER") {
//Change your Join type;
 $table['join']-&gt;type = 'LEFT';
   }
 }
}