You MUST have a security certificate (SSL) installed on your server before setting this. You can ask your hosting provider help you install SSL or buy the service from us here
When you have ssl Cer installed correctly at your server, please continue below steps:
1. Enable ssl mode: Admin panel -> System settings -> Security tab -> Check “Enable SSL Mode” and save changes.
2. Change .htaccess file at your site to make it auto redirect to https
3. Test your site by enter https://yourdomain.com to see if it works or not.
Hope it helps.