Open up a terminal.
Suppose : xx:xx:xx:xx:xx:xx = new mac address you wan to assign to you box
example: 1a:2b:3c:4d:5e:6f
Characters allowed in mac address: 1 2 3 4 5 6 7 8 9 0 a b c d e f
enter the following:

$ sudo ifconfig eth0 down
$ sudo ifconfig eth0 hw ether 1a:2b:3c:4d:5e:6f
$ sudo ifconfig etho up

Check the new MAC address by using following command

$ ifconfig