You can find your username in 2 different places. One place is right under your email address in your profile settings. Another place to find your username is listed at the bottom of the page when you click into “Find Friends.” It’s a combination of your name and numbers, so it might look something like “logan12345.”
If you want to change your username, tap on Profile and then the gear in the upper left corner. Tap on the Account section, and you'll see a place to edit your username.
(Example from iOS)