Usually the username doesn't have to be changed during an active working contract. If you do need to change it, it can be done in Employees-section.
Please note that the username must be unique, for example a combination of first and last names.
1. Go to Employees - Persons.
2. From the magnifying glass open up the correct person's person card.
3. Scroll down to Identifiers.
4. Type in the username field the new username. Please note that the field can be managed with validity tool.