For the far majority of customers, you can edit your hostname using the File Editor on the Control Panel!

Otherwise, please read the below:

Changing the hostname
To edit your server name please do the following:
a) Open up your FTP program and connect to FTP using the details provided.
b) Edit the line beginning hostname in your server config file. With the majority of game servers, this will be set at the top.
Remember that you are not allowed to remove the '!! INX-Gaming.co.uk >>' branding from the server hostname unless you have paid for debranding.
c) Change the map/level on your server to see these changes come into effect.

If you are a Control Panel customer, your config file location is displayed on the home page when you log in.

Finding your config file
CS Source: /srcds_l/cstrike/cfg/server.cfg
CS 1.6: /hlds_l/cstrike/server.cfg
DOD: /hlds_l/dod/server.cfg
DODS: /srcds_l/dods/cfg/server.cfg