1. Open Yahoo Messenger:
Run the main yahoo! messenger window and sign in.
2. Goto:
In main Yahoo! messenger window, top menu, goto (as shown in the image below):
(Top Menu) > Messenger > My Contact Details
2. In the options window that pops up, simply fill / change the values for your name for the Yahoo! messenger name (as shown in the following image).
there, when you enter the new name, first, middle and last name, it will be shown as such in the chat to the contacts and you too. If you want to show your name to yourself differently, then enter a Nickname to you too
3. Save and Close:
Once you have entered the new name and nickname (if required), simply click Save and Close, and the new name will be used from now on to be shown to your contacts on Yahoo! messenger chat.