If you wish to change your username, you can buy such an offer in the Coin Market.
On the other hand, nothing is stopping you from creating a new account with the desired username.
You can always delete your old account after making a new one. Even if you have forgotten your password, you can remove the account. Just contact our Support Team so that they can help you out!
If your previous account is connected with some social media account (like Twitter, Facebook or VK), creating a new account is no problem. Simply, disconnect your social network from your old profile and connect it with the new one.