It’s generally a Bad Idea to change usernames. If it’s changed, previous referrals to that user break. That’s why it’s so short.
Of course, you may want to change it.
Is that’s the case, please inform (new username) to some of the moderators, and they will change it for you.
They will change it for you, and fix any broken shortcut if you was eventually mentioned in some conversations.
Note: All the other information on the profile can be accessed and changed by you!