You need to change addresses into following files:
- /etc/network/interfaces
- /etc/resolv.conf
The following article from www.howtogeek.com prefers vi editor, but I use nano editor.
sudo nano /etc/network/interfaces
change to something like this
iface eth0 inet static
     address 192.168.1.5
     netmask 255.255.255.0
     network 192.168.1.0
     broadcast 192.168.1.255
     gateway 192.168.1.1
and DNS settings
sudo nano /etc/resolv.conf
and then restart
sudo /etc/init.d/networking restart
If you have DHCP client enabled, you must disable it. Consult the following article for more details: