From the Community Standards & Rules Announcement:
USERNAME CHANGES
Users cannot change their own account’s usernames. You may make a request to the administrators in the Name Change Request thread on the Q&A forum, but understand that they are under no obligation to fulfill your request as they have more important site issues that occupy their time. Requests made within a year of a previous request will be ignored. Creating a multiple account to circumvent this system may result in a ban from the site.