On our desktop website, your username appears in the header of each page beside your avatar (and when viewing your profile, it’s in the URL following letterboxd.com/). In our apps, open the action sheet on your profile, and you’ll see your username below your display name.
If you need further assistance, and you know your account email address, head here and provide it, and you’ll receive an email that includes your username. (There is no need to reset your password in this case, you can simply ignore the email.)