When you are doing network pentesting  it is important that you change your mac address or if you are doing anything illegal like wifi cracking and wifi have some extra security like mac filtering. So in that case you have change you mac address.
To change you mac address you need any linux operating system like kali linux, parrot operating system, ubuntu.

Before you change your mac network address plug you network interface to you linux machine. Now open you terminal and enter the following commands.

ifconfig wlan0 down

Now with this command your network interface will be down and you won't be able to connect to any network. before changing your mac sure your network interface is down.

ifconfig wlan0 hw ether [ and your new mac address]

Enter your new mac address or the mac address you want with the following command

ifconfig wlan0 up

As soon as you change your mac address you have to up your network card so you can connect to network

Now if you type ifconfig and you can see your mac address will be changed to the new mac address which you have specified from the above command