Go to http://www.fourleaflondon.com/wp-login.php and login. You may see the warning on the top with every page you visit, ignore it.
Go to Dashboard > Updates, tick the Simple Fullscreen Responsive Slider plugin and click Update Plugins.
You can also do this via FTP.
1. Download the plugin to your computer and extract it.
2. Navigate to public_html/wp-content/plugins/ and rename the directory simple-fullscreen-responsive-slider to simple-fullscreen-responsive-slider_old.
3. Upload the extracted directory (in step 1) along with its contents to public_html/wp-content/plugins/