How to change Webform field value after a webform has been submitted
You should make a custom module, rather than using Rules. 

Your module will implement the hook_webform_submission_presave, as shown also in webform.api.php inside webform directory.

Your function should look like this (well, this could be your entire .module file):

function mymodule_webform_submission_presave($node, &amp;$submission) {
  $component_id = 3; //that's your hidden field
  $submission-&gt;data[$component_id]['value'][0] = 'Yes';
}


Making a custom module is very easy, take a look here.