Hi Ahcene,
If you created your brother's profile then you can change his name. When you create a person on WikiTree you become the 'Profile Manager' which means you can change much of the person's information.
On your brother's profile near the top of the screen is a row of GREEN boxes. Click on EDIT and you will then be able to change his name.
You should also enter a reason why you are changing things in the box "Explain your changes".
Don't forget to scroll down the page and click on SAVE CHANGES after you change things!
Good Luck!
Kevin