How to embed webform to update existing submission
Existing webform submissions are entities and you can use the entity form builder service to build the form:

// load the webform submission entity
$webformSubmission = WebformSubmission::load($submissionId);
// build a form for the entity
$webform = \Drupal::service('entity.form_builder')-&gt;getForm($webformSubmission, 'edit');