Hover the cursor over your username, in the top right corner of the page.
A dropdown list will appear directly below your username.
Click the ‘account’ option on this list.
You will be taken to a page displaying a number of account options. On the left-hand side, you will see a section entitled ‘Change Username’.
Click in the box containing your username, delete your username from the box and type a new one.
Click ‘Change Username’