In order to change your user name you need to go to your profile and click on the pencil icon that is next to your username (as in the screenshot below), and change it. Please pay attention to the fact that you can only change your username once every 24 hours, so if you already did it today, you can try tomorrow.
If you need it urgently and already changed it today, please let us know and we will change it for you.
Kind regards,
Humanz support.