This answer is relevant if your website has been designed and built by BT. View FAQs for Web Hosting if you built your website. If you're not sure which website package you have, please read How to get support for your website.
Login to your Control Panel, (i.e. go to www.yourdomain.com/siteadmin - but replace 'yourdomain.com' with your website address), and click on 'Page editor'. From here, you'll be able to update the title and description of the page.