gw-gravity-forms-edit-entry.php

<?php
/**
 * Gravity Wiz // Gravity Forms // Edit Entry
 *
 * Edit entry ID specified in field with current form submission.
 *
 * @version   1.0
 * @author    David Smith <david@gravitywiz.com>
 * @license   GPL-2.0+
 * @link      http://gravitywiz.com/...
 * @copyright 2019 Gravity Wiz
 */
// Update "123" with your form ID; Update "456" with your field ID.

add_filter( 'gform_entry_id_pre_save_lead_123', 'my_update_entry_on_form_submission', 10, 2 );

function my_update_entry_on_form_submission( $entry_id, $form ) {

	$update_entry_id = rgpost( 'input_456' );

	return $update_entry_id ? $update_entry_id : $entry_id;

}