Suppose we will be updating a field named 'confirmed' in a model called 'User' with the id of 5. We will then have an array with just that value, like this:
$data = array(
'User' => array(
'id' => 5,
'confirmed' => 1
We should then pass it on to a save function:
$this->User->save( $data, false, array('confirmed') );
You can RTFM about the save() function here.