Changing the hostname without restart
To change your hostname first we need to edit the following file:
/etc/hostname and set the new hostname.
sudo nano /etc/hostname
After that we issue the following command in a terminal:
sudo hostname new-hostname
This will set the new hostname until you restart. See man hostname and How do I change the computer name? for further information.
After a restart your changes in /etc/hostname will be used (that we already edited)
We should also update the /etc/hosts file and change the line which reads:
127.0.1.1 old-hostname
so that it now contains your new hostname.