I can see you've written 21 hubs and have been here for three months.
Unfortunately, you can't change your user name for the same profile but what you can do is create a new profile with your desired user name.
You can then delete as many hubs from your current account as you wish (may be you can think about the recent ones) and publish them through your new account.
That way you can retain your hubs too and enjoy your new user name also.
It's only been three months yet and when it's necessary, it's actually beneficial in the long run.