From the Dashboard, click on the User Profile (this may be your initials, picture, or avatar in a circle) from the top right-hand side of the screen. 

Then click on the Profile option.  From the Settings screen, click on the Security setting under the General heading.  A user may edit his/her Username at any time, by clicking on the ‘Edit’ function.

A new Username, must not contain a space, must contain 1 or more letters, the ID cannot be part of the current password, the ID must be between 4 and 12 characters long.

Be sure to save your changes after you edit.