follow these steps:
[admin@MikroTik] /interface ethernet> print
Flags: X - disabled, R - running, S - slave
# NAME MTU MAC-ADDRESS ARP MASTER-PORT SWITCH
0 R ether1-g... 1500 00:0C:42:56:98:14 enabled
1 R ether2-l... 1500 00:0C:42:56:98:15 enabled none 0
2 S ether3-l... 1500 00:0C:42:56:98:16 enabled ether2-local-ma... 0
3 S ether4-l... 1500 00:0C:42:56:98:17 enabled ether2-local-ma... 0
4 S ether5-l... 1500 00:0C:42:56:98:18 enabled ether2-local-ma... 0
[admin@MikroTik] /interface ethernet> set 4 mac-address=00:0C:42:56:98:66
[admin@MikroTik] /interface ethernet> print
Flags: X - disabled, R - running, S - slave
# NAME MTU MAC-ADDRESS ARP MASTER-PORT SWITCH
0 R ether1-g... 1500 00:0C:42:56:98:14 enabled
1 R ether2-l... 1500 00:0C:42:56:98:15 enabled none 0
2 S ether3-l... 1500 00:0C:42:56:98:16 enabled ether2-local-ma... 0
3 S ether4-l... 1500 00:0C:42:56:98:17 enabled ether2-local-ma... 0
4 S ether5-l... 1500 00:0C:42:56:98:66 enabled ether2-local-ma... 0
[admin@MikroTik] /interface ethernet>