All about.me pages are equipped with SSL certificates. If you chose a domain from about.me during your upgrade, your domain is already secure.
If you mapped a domain you already own, please go to about.me/user/edit/domains/dns to make sure you've entered the DNS records correctly at your domain registrar.
Please note: HTTPS will not work with www.yourdomain.me. If you have www in your CNAME (i.e. www.yourdomain.me), your shareable link is either www.yourdomain.meor https://yourdomain.me