The default web server identification string of your OpenVPN Server can be one way to identify it in automated vulnerability scanners such as nmap.

To change it to an arbitrary value to reduce the chance of being identified by vulnerability scanners, you can do the following (as root or sudoer):

cd /usr/local/openvpn_as/scripts
./sacli --key "cs.web_server_name" --value "BLABLABLA" ConfigPut
./sacli start

Source: https://openvpn.net/vpn-server-resources/managing-settings-for-the-web-services-from-the-command-line/

That’s it, Enjoy!