You can easily redirect your domain from HTTP to HTTPS by clicking on "Actions" > "Misc" > "Domain Redirects" and turn "Redirect HTTP to HTTPS" on. Update takes about 5 minutes.

If you want to use .htaccess, follow the guide below.

Place the following code to your .htaccess file which you can find from your domain's root directory:

RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^.*${REQUEST_URI} [R,L]

If RewriteEngine On already exists in your .htaccess file, you may remove it.