To change your username you must access the Settings menu by clicking on the gear icon in the upper right corner of your screen. When entering, you must select the first option that contains your profile and avatar information to access your account.
From the "My account" menu, click on the "User" option to edit it. Remember that your new username must be unique. To finish, select "Save".
Please note that you will only be able to edit this information if you have created your account using an email address. In case you have linked your account to your Facebook profile, you will not have the possibility to change your username.