Pick one or more alternative names and check the member list to make sure they are not already taken, then send an email to sitesupport (at) CrackBerry.com. Give a reason for wanting the change.
EDIT: corrected the email address.
Posted from my CrackBerry at wapforums.crackberry.com
Avatars, User Names, Signatures, General Posting Guidelines
Member User Names and Avatars must be appropriate for a Public Forum of all ages. If the moderation team does not feel your Avatar or User Name is appropriate you will kindly be asked to change it (User Name will be changed to one of your choosing by Administration). Username changes can be made by an Executive moderator. Username changes are only granted in the case where a member accidentally registers with an inappropriate username (something offensive that must be changed, a full name or email address) or a special circumstance requires a change absolutely necessary. You may contact sitesupport @ crackberry.com to request this change.