You cannot change a WordPress.com account username once it has been set. However, you can change what is seen on your posts and comments by changing the display name (nickname) in your profile, or you can create a new account and transfer your blog(s) to the new account.
How to wordpress change admin username
wordpress change admin username
Change Display Name (Nickname)
1. Access your User Profile
2. Change the value next to Display name publicly as under Basic Details
3. Click Update Profile.
↑ Table of Contents ↑
1. Log into your current account (using your existing username)
2. Access your Personal Settings and change the e-mail address to another email address. This will free up your primary e-mail for use with the new username.
3. Once the new address has been verified via email confirmation, log out of the account.
4. Go to http://wordpress.com/signup/ and attempt to register a new account (the username you wish to change to) with your e-mail address.
5. If you’ve got a blog you should transfer its ownership to your new username.
Once you’ve done all of that you’ll be rocking WordPress.com with your new username.
via Change Your Username « Support.