You will have to change the hostname in the following files -

/etc/hosts
/etc/postfix/main.cf
/home/admispconfig/ispconfig/lib/config.inc.php
/root/ispconfig/httpd/conf/httpd.conf
/etc/hostname

Then restart ISPConfig via '/etc/init.d/ispconfig_server restart'

..and then log into the ISPConfig control panel and navigate to management > server > settings and change the hostname there as well.