You need to take everything from inside silverstripe/ and put it all a directory back, in public_html
If you have SSH access, you could login and go
sudo mv /var/www/public_html/silverstripe/* /var/www/public_html/
sudo mv /var/www/public_html/silverstripe/.htaccess /var/www/public_html/
(assuming your webroot is /var/www/public_html)
You will then need to edit your .htaccess file to remove the /silverstripe part of your domain name.
If you are new to all this, may be easier for you to delete everything, and just upload it again in to public_html instead of a folder inside public_html.