How to clear selected values from multiple select field

I have a form with a multiple select field where a user can select several values in it

$form['export_type_section']['export_type']  = array(
  '#title' => t('Export types'),
  '#type' => 'select',
  '#multiple' => TRUE,
  '#size' => 10, 
  '#options' => $options,
  '#ajax' => array(
    'event' => 'change',
    'callback' => 'test_export_form_export_type_ajax_callback',
  ), 
);

I have another field in the form

$form['document_type'] = array(
  '#type' => 'select',
  '#options' => array(0 => t('All Types'), 1 => t('Test1'), 2 => t('Test2'), 3 => t('Test3')),
  '#title' => 'Document Types',
  '#default' => 0,
  '#ajax' => array(
    'event' => 'change',
    'callback' => 'test_export_form_document_type_ajax_callback',
  ),
);

I am trying to figure out how to reset the export_type field when the document_type is changed. I would like to clear the selected values so that the user can reselect what is needed whenever the document_type changes. I tried several formats in the test_export_form_document_type_ajax_callback but none worked.

Anybody knows how to do it?