How to change your username:

PC/Mac:

Log in with your username and your password
Click on 'My Profile'
Now click on the button 'Edit Profile'
Here you can see your username and below the button 'change'
Click on it
Enter your new username and repeat it in the box below.
Then click the green button to submit.


Mobile Device

Click the three lines in the top left corner
a menu opens
Select 'My Profile'.
Scroll down 
Here you can see your username and below it, the button 'change' 
Enter your new username and repeat it in the box below 
Then click the green button to submit.

You can only change your username only once a year!

Kind Regards,
John Stewart