Here is the tutorial to change twitter username
  - Go to "Settings and privacy" from your  profile icon drop-down menu.
- Under Account, click the text box beside  Username and type a new username in the space. If the username is taken,  Twitter will prompt you to make a new one. If the username is available, you'll  see a green "Available!" notification.
- Click "Save changes" at the bottom of  the screen.
- Go to "Settings and privacy" from your  profile icon drop-down menu.
- Under Account, click the text box beside  "Username" and type a new username in the space. If the username is  taken, Twitter will prompt you to make a new one. If the username is available,  you'll see a green "Available!" notification.
- Click "Save changes" at the bottom of  the screen.