By default changing MAC address of ethernet interface in Mikrotik Winbox is disabled:
But You can very simple change MAC in terminal.
In left side menu select New Terminal:
/interface ethernet set ether1 mac-address=XX:XX:XX:XX:XX:XX
Where ether1 is required interface and XX:XX:XX:XX:XX:XX is new MAC address of your device.
Credit: https://andjey.info/how-to-change-mac-address-on-mikrotik-routerboard/