GitHub is partnered free certificate authority “Let’s Encrypt”. Nowadays, custom domains on GitHub Pages are supporting for HTTPS. You have to update your DNS records with new IP address.
Once your DNS record is propagated, your site will be work accurately with HTTPS protocol. Now you should select “Enforce HTTPS” option under your repository’s settings to redirect your traffic from HTTP to HTTPS.
For more information - https://blog.github.com/2018-05-01-github-pages-custom-domains-https/