If you want to update some fields programmatically, you must know node_save() and field_attach_update() functions.

Node Save (node_save()):

Create a new node or update existing node will use the node_save() function in Drupal 7 but when used node_save() to update filed it will update time stamp and other fields. Maybe this is won’t safe to update other fields for single update field by using node_save() but we can approach better by using field_attach_update() to update exits field.

field_attach_update()

field_attach_update() will not change any time stamp or any other hook that node_save usually invokes and field_attach_update saves the field data for an existing entity. It saves the field data value instead of the entire node

Simple copy below code and change field name and value according to your code.

<?php
$node = node_load($nid);
$node->field_fieldname[LANGUAGE_NONE][0][‘value’] = ‘some value to update’;
field_attach_update(‘node’, $node);
?>

Multiple Fields Update

If you want to update multiple fields at a time for the specific content type by using field_attach_update(). Follow below example for more clarity.

Here simple example to update all program ids field for a specific content type. In below, an example will load all program ids which is related to program content type

<?php
$programNodes = node_load_multiple(array(), array(‘type’ =>’program’));
foreach ($programNodes as $programID) {
$programID->field_fieldname[LANGUAGE_NONE][0][‘value’] = ‘New Value’;
field_attach_update(‘node’, $programID);
}
?>