Log in to Twitter.com and visit your account settings pages from the dropdown menu in the top right corner.
On your account settings page, change the username listed in your username field.
If the username is taken, you'll be prompted to choose another one.
Click Save changes at the bottom of the page.
NOTE: Changing your username will not affect your existing followers, direct messages, or @replies. Your followers will simply see a new username next to your profile photo when you update. We suggest you alert your followers before you change your username so they can direct @replies or direct messages to your new username.
What's the difference between your username and your name?
Your username appears in your profile URL and is unique to you. It's used for logging in, @replies, and direct messages.
Your name is a personal identifier (sometimes a business name or real name) displayed in your profile page and used to identify you to friends, especially if your username is something mysterious like @neongolden or @meangrape. Change your real name in your profile settings tab.