Update a node object and save it (Drupal 7)

update_node.php

<?php
// Load a node with nid = 1000.
$node_nid = 1000;
$node = node_load($node_nid);

// Change the title of the node.
$node->title = 'New title';

// Update the value of a field with the language property of the node object.
$node->field_custom[$node->$language][0]['value'] = 'New custom field data';

// Set the revions property and save the updated node.
$node->revision = TRUE;
node_save($node);