This forum is for 3rd party Twitch developers, in the future for Twitch support you should use https://help.twitch.tv/
To answer your question though you can change your username as long as the username you’re trying to change to is available, and provided you haven’t changed your username already in the last 60 days. There’s more info on username changes here: https://help.twitch.tv/customer/en/portal/articles/1015624