1] For changing mac address to 11:12:43:44:75:56 (let say)
2] Just type -
macchanger --mac 11:12:43:44:75:56 eth0
3] eth0 is the interface name.
Use ifconfig command to check primary interface card.
4] get back the original mac address
This spoof mac address is not permanent, original mac is restored after reboot.
Without reboot, for getting back to original mac address-
macchanger -s eth0
(this will list the original mac address of eth0 interface)
macchanger --mac originalmac eth0
(it will set the original mac address)