New users can change their username within 3 days of registration. 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 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.