Why when I go to my account and change my screen name, my old name keeps coming up. How do I change it?

I want a new screen name, how do I change it, I put it in my profile, i sign in with it, but my old name shows up Online. Can someone tell me how to change it.Why when I go to my account and change my screen name, my old name keeps coming up. How do I change it?
Go to your profile by clicking on your avatar, click ';edit my info';, go to ';nickname';, mark ';use another nickname';, type it in the box. You have 22 characters to work with. Click ';preview'; and then ';okay';. The new screen name will appear underneath your avatar from now on...:-)