Edit your .htaccess file to add the following line:
Header always set Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
The above line will enable HTTP Strict Transport Security (HSTS) on your website. This will tell browsers to load your website & its subdomains using HTTPS. Here I have set the max-age to 365 days. You can change the value as per your wish.
If you stop using HTTPS on your website, browsers will give error whenever a user will try to access your website. So, make sure you continue supporting HTTPS on your website after enabling HSTS..