Hook_Form_Alter Edit Existing Field Type
hook_form_alter work only with module implementation. So you have to use it in any module not in template.php

Use the above statement in module like 

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {

  if($form_id == "views_exposed_form"){
    $form['data_2'] = array(
            '#type' =&gt; 'date_popup',
            '#date_format' =&gt; 'Y-m-d'
        );
    }
}