In the bottom left corner, click on your instance name to expand, then choose settings. 

In the Dashboard Admins section which appears in the right hand panel, click on the Name of the user. 

To delete the user, simply click "Delete this user"

You must have AT LEAST ONE Super Admin. If you click on the name of someone who is the only Super Admin, the delete option will not be available for that use. Click into another user and choose "MAKE SUPER ADMIN". Then, you can go back and delete the other user.