In order to change your Username, first, make sure you have completed 1 year since last time you changed it.
1- Click on your Profile Picture/Dashboard and click on the red Settings button
2-Sign in with your UN and password
3- Now, Under the 'Profile', You'll see your Username above the 'Change Username' (If you completed 1 year or more)
Make sure to follow the Username Policy listed down
Click on Save and you are done!