To change the staff member's username you would delete it and add a new one.
Go to the staff member's profile (Admin > Personnel > Staff > select staff member) and click change password/roles, next to where the photo shows. In this area click "Delete SP logon" at the base.
This will then return to the front page of the staff profile. Click create to add the new username. Enter the username, password, select the required roles and click save.
To do this you will need the admin and sysadmin user roles.
Please be advised if you are doing this on behalf of yourself you will need to find another Admin and Sysadmin user in the school to assist, as your permissions will temporarily be withdrawn until you have a new login set up.