When you change your Twitter username, the change to your account is instant and your old username immediately returns to circulation for use by other Twitter users. You can't change your username through the Twitter app. Instead, you must log in to your account on the Web and change it through your account settings.

How to Change Your Twitter Username

After logging in to your Twitter account, click the gear icon in the Twitter home page and select the "Settings" tab in the drop-down menu. Type the new username that you want in the Username field and wait a few seconds. Twitter checks whether the new username is available for you to register. If it is, press "Enter" or click the "Save Changes" button to register your new username. Enter your account password in the pop-up dialog box that launches, and then click "Save Changes" to confirm the change. The change to your account processes almost immediately.