Views hook query alter, change the DatabaseCondition Object
The conditions may be altered by getting them by reference:

function custom_views_query_alter(&amp;$view, &amp;$query) {
   $conditions = &amp;$query-&gt;conditions();
   foreach($conditions as $idx =&gt; $condition) {
     if ($condition['field'] == 'node.type') {
       $conditions[$idx]['value'] = array("Some other type");
     }
   } 
}


Don't forget to modify the object/arrays by reference with the &amp; character.