After retrieving an entry, you can update that entry and save changes back to the server. Here's an example:
<?php
$client = Zend_Gdata_ClientLogin::getHttpClient($user, $pass, $service);
$gdata = new Zend_Gdata($client);
$query = new Zend_Gdata_Query(
'http://www.blogger.com/feeds/blogID/posts/default');
$query->setMaxResults(10);
$feed = $gdata->getFeed($query);
foreach ($feed as $entry) {
// update the title to append 'NEW'
echo 'Old Title: ' . $entry->title->text;
$entry->title->text = $entry->title->text . ' NEW';
// update the entry on the server
$newEntry = $entry->save();
echo 'New Title: ' . $newEntry->title->text;
}