<?php
require_once('app/Mage.php');
umask(0);
Mage::app();

$submissions = Mage::getModel('plugincompany_contactforms/entry')->getCollection();

$submissions
    ->addFieldToFilter('form_id', 1) //example parent form id filter
    ->addFieldToFilter('created_at', ['gt' => '2017-10-15']) //example submission date filter
;

foreach($submissions as $submission) {
    //set status to 'viewed'
    // you can see all statuses here:
    // Plugincompany_Contactforms_Model_Entry_Source_Status
    $submission->setStatus(2);
    $submission->save();
}