Views: Change the text of the Apply button of the exposed filters

In the template.php file of your theme, add the following code:

function YOUR_THEME_preprocess_views_exposed_form(&$vars, $hook)
{
  /**
    Only alter a specific exposed filter form
    Note: You need to change the form id to match yours.
  */
  if ($vars['form']['#id'] == 'views-exposed-form-YOUR_VIEW-views-page-1') {
 
    // Change the text on the submit button
    $vars['form']['submit']['#value'] = t('Search');
 
    // Rebuild the rendered version (submit button, rest remains unchanged)
    unset($vars['form']['submit']['#printed']);
    $vars['button'] = drupal_render($vars['form']['submit']);
  }
}