The same procedure is used to update an existing index. The only difference is that the open() method is called instead of the create() method:
<?php
// Open existing index
$index = Zend_Search_Lucene::open('/data/my-index');
$doc = new Zend_Search_Lucene_Document();
// Store document URL to identify it in search result.
$doc->addField(Zend_Search_Lucene_Field::Text('url', $docUrl));
// Index document content
$doc->addField(Zend_Search_Lucene_Field::UnStored('contents',
$docContent));
// Add document to the index.
$index->addDocument($doc);