So it is a case of moving the identity. (Cloning).
You need to supply much more information.
The hardware you have, model, partnumber, links to the spec.
You can usual change the MAC, but it not always easy and offen required hacking the MAC PHY EEPROM.
You need to first find the Ethernet Controller, there is uaually an EEPROM next to it. Something Like 93CXX. Sometimes tools are avaliable from the vendor of the Controller that allow you to reprogram the EEPROM. Oherwise you need to directly connect to it, which may required removing it and use a EEPROM reader/progammer.
You then need to find the MAC in the EEPROM memory and alter it.
The big problem here is the chip may also have a CRC to protect againist Corruption or alteration. If I does, you will need to recreate the CRC. This can take some working out.
Regards
Ian