how do i change my username???:(
It's always worth looking in the help section of the website for these sorts of questions. There's a link at the bottom of every Duolingo page, and in the menu that appears when you hold your mouse over the username at the top of the page.
Have a look at this article from the help section: https://support.duolingo.com/hc/en-us/articles/208606896-How-do-I-change-my-username-or-email-