How to use hook_webform_submission_presave
Notice the value on which you check in the if.

the correct code is:

function webform_presave_webform_submission_presave($node, &amp;$submission) {
  if ($submission-&gt;nid == 28) {
    $component_id = 1;
    $submission-&gt;data[$component_id][0] = 'Yes';
  }
}