You can update your username in-app from your Profile.
Note: You can only change your username every 14 days, so if you’ve made an update recently and cannot change it now, try again later! If you’re not seeing a recent change, fully close the app, and open it again.
To update your username:
Tap your profile pic at the top right.
Tap your username below your name (ex @username).
Update your username.