Usernames are permanent and cannot be changed.
If you want a different username, you may open a new Photobucket account by clicking here. You will need to use a different email address for the new account, one that is not currently tied to a Photobucket account as we only allow one Photobucket account per email address.
Also, we do not have a way to transfer the content of one account to another. This can only be done by downloading the images from the old account, then uploading them to the new account.