Here are some better instructions as to how to change your username. Step 1
Click on the down arrow located on the top right hand side of the menu, underneath the search bar. This should bring up a menu, where you can then select "Profile"
You should then be taken to a page with a box on the left hand side with your current username. Click on the "Edit" button below your username
Once you've clicked edit, you should be able to type over whatever name has been automatically created before. After you've entered your name, make sure you click "Save"!
I hope these set of instructions are helpful - as always, please contact me at email@example.com if you need any assistance!