I can'y log in to change my settingd

hello everyone,

Once i wanted to change my username of my macbook en did that but after that i've not been able to to open the little slot in the setting because they always say i have entered the wrong username or password. It's not the wrong password because if i shut down my laptop en start it they ask my password en its ok, so probably is the username. But what is my username, if i connect my laptop with a device it shows "Macbook air van Landry" but if i go to settings it just shows "Macbook". i've tried both but non of them works. How can i fix this