NXS -- screen name changes are discussed in the Answers to common questions
thread in the Site Suggestions and Help forum. Here's the relevant text:
QUESTION: Can I change my username?
ANSWER: Maybe. We typically don't allow username changes, as it results in a lot of confusion. However, if you've accidentally misspelled your username, or if you unintentionally entered your real name as your username, then we're usually happy to change your username for you. To request a username change, please start a new thread here in the Site Suggestions and Help forum.