1. First step is to find the current MAC address and take a backup copy in a safe place.
To know your current MAC type:
$ ifconfig | grep HWaddr
eth0 Link encap:Ethernet HWaddr 00:19:48:2E:D1:27
The above hexadecimal numbers 00:19:48:2E:D1:27 is your MAC address for device eth0.
Simply copy and paste it in a file.
2. Now, time to change the MAC address this requires you to work as root user.
Take the network interface down
# ifdown eth0
Now, lets change the current MAC address to 00:E0:81:5C:B4:0F
# ifconfig eth0 hw ether 00:E0:81:5C:B4:0F
Take the network interface up again
# ifconfig eth0 up
Check if your MAC address is changed:
# ifconfig eth0 | grep HWaddr
Changing MAC address is only temporary. Once you reboot your machine, the operating system will reflect the physical MAC address burnt in your network card and not the MAC address you set.