mepr-password-reset-subject.php

<?php
function mepr_change_subject($subject, $recipients, $message, $headers) {
  if(strpos(strtolower($subject), 'password reset') !== false) {
    $subject = 'New Subject - Password Reset';
  }
  
  return $subject;
}
add_filter('mepr-wp-mail-subject', 'mepr_change_subject', 10, 4);