In order to modify this page, login to your server via ssh and cd over to /usr/local/apache/htdocs
cd /usr/local/apache/htdocs
Here, you can edit the file index.html
vim index.html
If you list the content of /usr/local/apache/htdocs you will see that you can also modify the content of other error pages that apache outputs as well.
Also, you can also edit these pages live using komposer using the Edit >> Publishing Site Settings and connecting to the server via ftp.
