This should do the trick:

RewriteEngine On 
RewriteCond %{SERVER_PORT} 80 [OR]
RewriteCond %{HTTP_HOST} ^example.com [NC]
RewriteRule ^(.*)$ https://www.example.com/$1 [R,L]

This will redirect any URL not using the www subdomain and any URL that is not https:// to the https://www version of your site.

Apache does not have mod_rewrite enabled by default so for this to work you’ll need to do two things.

1) Enable mod_rewrite

sudo a2enmod rewrite;
sudo service apache2 restart;

2) In your /etc/apache2/sites-enabled/ config file you will need to make sure that the <Directory> block for your web root has the option AllowOverride All set. This tells Apache it can use directives in an .htaccess file to override the default settings.

After changing the file, restart Apache:

sudo service apache2 restart;