update_file.php

<?php

// Method 1: Retrieve file details using HEAD
$file = $container->getObject('foo.jpg');

// Method 2: Only set the name (which is the only requirement for an update)
$file = $container->dataObject();
$file->setName('foo.jpg');

// Now update content
$handle = fopen('/path/to/new/content', 'r+');
$file->setContent($handle);
$file->update();
fclose($handle);