For anyone else with this question, Frogabog is spot on - just send a request to MODX Cloud support (from the right hand Support link in the dashboard, or
[email protected]).
What crushedknuckles wants is a change to the "account code", and support can easily take care of that for you. Our signup process uses your chosen username for your account code, to make the signup process simpler. But behind the scenes your username is separate from your account code. So when we change your account code, your clouds' URLs will change, but your username will not.