Changing the WebGUI port

Hello you nice people!

I have a lot of luck beeing able to have a hardware server in a very fast network. The server is located behind a firewall.
The problem: I can not access the server GUI, because the ports are beeing blocked from the firewall.
There is absolutely no chance to have these ports opened, so i have to change the ports.
How can i change them?

Regards,

WatchDog