If you don't want to reboot your computer, then become root and type this:
[arnav@dhcppc1 ~]$ hostname lappy
then exit your terminal and open a new terminal.
The actual file to change your hostname is:
[arnav@dhcppc1 ~]$ vi /etc/sysconfig/network
If you edited file /etc/sysconfig/network, then you need to reboot your computer.