Add the following two lines of code into the .htaccess file:
RewriteCond %{SERVER_PORT} !^443$
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
WARNING: For testing purposes, you might want to remove the [L,R=301] condition from the second line and instead just use [L,R]. R=301 will make your browser PERMANENTLY redirect to the new URL and there's no way you can break out of that if you missconfigure your .htaccess file! You might get around the mistake, but your users WON'T!
Your final .htaccess should look like this:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
RewriteCond %{SERVER_PORT} !^443$
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R]
</IfModule>
# END WordPress