from the command line as root or su
iface eth0 inet static
dns-nameservers 192.168.1.1 126.96.36.199
First change your adapter from dhcp to static.
Enter the information as above with your ip address settings.
crtl-x in nano and overwrite interfaces file.
restart networking with this command. (if over ssh make sure to adjust your ssh connection to the new ip address)