Tap on the icon with three stacked horizontal lines at the very top right corner of the screen.
Under Preferences on the Settings screen, you will see the first option, which is Edit Profile.
Tap on Edit Profile.
On this screen, you will see the third option labeled as Username. You should be able to change your username by editing the particular name on that section.
Once you have changed your username, tap on Save at the very top right corner of the screen to save the changes.