Ubuntu Change Hostname

Reboot not required

hostnamectl set-hostname new-hostname

Reboot required

sudo vi /etc/hostname
 
# change the string

sudo vi /etc/hosts
 
# Example content:
# 127.0.0.1 localhost
# 127.0.1.1 ubuntu-machine <- change this string

 Then, reboot

Temporarily (the other approaches changes the hostname permanently)

sudo hostname new-hostname