Update a cck field value of a node
You can use node_save() to do this; CCK will automatically hook into it and update the field values in the database for you:

// Load a node
$node = node_load(1);

// Update the field value
$node-&gt;field_field_name = array(0 =&gt; array('value' =&gt; 'A value'));

// Save the node. Field values are also saved, and the node modified time is updated.
node_save($node);