From your GitHub home page, select the drop-down menu on the right and select Settings.

Next, click Account from the options in the sidebar on the left.

Hit change username to start the process.

Click the big red button to acknowledge you understand the risks.

Make sure your username is available, and click Change my username to finalize the changes.

Maybe you don't like your current GitHub username, or you just want to change it. Fortunately, changing your GitHub username is very easy. Beware, as changing your username can have unintended side effects like missing redirects for your old profile and repositories.

How to Change GitHub Username