Users can't personally change their usernames.
However, we can change your username if your ideal username is available.
One thing to keep in mind is that the tasks you've already been tagged in with your current usertag won't be updated to the new username and consequently won't appear under My Tasks anymore, although they will still be in the project to which they belong.
If you'd still like us to change the username for you, just send us a message. You can do so by opening your sidebar and clicking the Help & Support icon. Then, select Contact Support. In your message, include your top three username choices, in order of preference.
We'll change it to the first one that's available and message you when we're done.