Each user is able to pick their display name during the registration step. After that only Moderators or Community Managers have the ability to change user's display names. If you would like your display name changed you may send a private message to any of these users (they can be identified by their badge, mod or admin) indicating what you would like your display name to be.
The following restrictions apply for display names:
Must be unique
Contains only alphanumeric characters plus hyphen and underscore