Unfortunately forcing your WordPress site to use HTTPS is not as simple as updating your .htaccess(in most cases) but luckily there is a very simple solution and in most cases you do not need to do anything other than install a plugin.
Assuming that you already have the SSL installed on your site, simply install the following plugin:
After installing the plugin, activate it. It will then prompt you at the top of the wordpress admin side, if you are sure you want to force https on your site in which you confirm.
Your site should now load up using HTTPS, any traffic going to HTTP will be redirected to HTTPS. If not, feel free to open a ticket up so we can look into it further.