The MAC address of a computer is hard wired into the network card within it, and can not be changed. It has to be a unique number with information that identifies the make and model, otherwise it will not work on the Internet. If you were to change th address, and it conflicted with another card having that address, both would be locked out of the Internet.
There are ways to "spoof" a MAC address, that is force it to send out a different address. But even that is limited because it has to have certain numbers in place (such as the 2nd digit on your card) for the Internet to be able to knwo who to talk to it. So even with "spoofin" you are unable to totally change the number - and the change is not permanent.
If you need a different MAC address for some reason, you will need to get a different network card. Purchase a USB network card to use at that times when you want to conceal your real address. Or simply use a proxy server. That is a special service where you attach to a server, and then it creates a "fake" ip address, MAC, etc that you use to do your surfing.
But you can not change the MAC address of a network card. It is hard wired into the card.
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