Name of my MacBook Air

How to change Mac username

1. Open System Preferences.
2. Users & Groups.
3. Click unlock and enter your password.
4. Now Control-click or right-click the user that you want to rename.
5. Choose Advanced.
6. Change the name in the full name field.
7. Restart the computer for the changes to take effect.

Go to Apple menu > System Preferences > Sharing > Computer Name.

Your login name and Mac name is not the same. The login was established when you initially setup your Mac for the first time.