By default changing MAC address of ethernet interface in Mikrotik Winbox is disabled:
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But You can very simple change MAC in terminal.
In left side menu select New Terminal:
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Then enter command:
/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.
Thats all.
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