To change the hostname on a Linux system you need to change 2 configuration files:
/etc/hostname
/etc/hosts
Old:
#/etc/hostname
oldname
#/etc/hosts
127.0.0.1 localhost
200.200.200.100 oldname

# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
New:
#/etc/hostname
newname
#/etc/hosts
127.0.0.1 localhost
200.200.200.100 newname

# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
Now reboot the system for the changes to take place.