If you'd like to update your Community account's username, you can do so very easily by following the steps below:
Please note that if your preferred username is already taken, you'll see a message to edit it first.
You can refresh the page if you're having troubles seeing any changes right away to shake things up.
Keep in mind that updating this only affects the Community. If you'd like more information on updating your username in the Spotify app instead, just head to this Spotify Answer for more info.
Didn't help? Search for more Spotify Answers, or create a new thread and ask the Community.