New users can change their username within 3 days of registration at no charge. One caveat is that old posts with mentions and quotes will have the wrong name embedded (which is why we limit the change period to only 3 days).
If your account is older than 3 days, you can simply create a new account (at no charge) using a different email address. Content associated with your old username will not be associated with your new username. We can merge the two, but that involves a professional techi’s time. As of Aug 2018, the price for this is $50.