You can update your username in-app from your Profile.
To update your username:
Tap your profile pic at the top right.
Tap your username below your name (ex @username).
Update your username.
Note: There is a limit to how often you can update your username, so if you’ve made an update recently and are unable to change it now, try again later! If you’re not seeing a recent change, fully close the app, and open it again.