How can i change user name?

[mayoreame]

Good Day! Sorry for my question but How can i change user name? I told my brother about this forum and he's register here. Than I wanted to register and going to registration form... I see next note (something like): Your E-mail adress alredy registered or used...wherever. I've had a registered with his e-mail adress. Could you Help me? How can i change my brother's nick name and change his password? Sorry but i'm girl and don't know how do this... Bye!