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"
Step 2
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
Step 3
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 abbeypnewman@gmail.com if you need any assistance!