Here's how:

1. Log in to your Yahoo! Mail account.
2. Click your username in "Welcome, username" in the top-right part of the page.
3. Enter your password and click Continue.
4. Scroll down to the Address/Contact Information section and click Edit.
5. The name you entered in the "First name" field is the name displayed on the Yahoo! Mail Welcome page.
6. To change it, double-click to highlight the current name and then type a new name.
7. Click Finished in either the upper- or lower-right corner of the page, then again on the Account Information page. All done!

Your new Welcome name now appears when you login to Yahoo! Mail. If you don't see the new name, click the "Refresh" or "Reload" button on your browser's toolbar.