You can easily redirect your domain from HTTP to HTTPS by clicking on "Actions" > "Misc" > "Domain Redirects" and turning "Redirect HTTP to HTTPS" on. The 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 ^.*$ https://your-domain-goes-here.com%{REQUEST_URI} [R,L]
If RewriteEngine On already exists in your .htaccess file, you may remove it.