updating the Telegram preview cache

This answer comes from https://stackoverflow.com/questions/35268940/how-to-force-telegram-to-update-link-preview and worked great for me.

Send a Telegram message to @webpagebot and it will activate, inviting you to send up to 10 links for it to update the cache for. I’m not sure if that means 10 links allowed at once, or 10 links ever; but in any case, it worked for me. It took me four tries – once to the shortlink, once to the actual blog post, once to the parent domain, once to the header image that had changed. It’s that final one that worked for me.