To modify the username associated with your account, you will need to access your User Settings. To access the User Settings, click on your username in the upper right corner of the page.
Once you have selected your account to edit, you'll be taken into the Users application. To make any changes to your account, click on the Edit button.
On the next screen, you can update your user name by clicking into the text field, and making the necessary adjustment.
When you have finished making your changes, click the Save button at the bottom of the page to save them.
You can learn more about the options available by viewing the article for Updating your User Settings.