In principle, roles can always be changed up to their own hierarchical level. This means that branch managers can appoint other branch managers and department heads can appoint other department heads. Employees with the role of "employee" cannot make any changes to their rights. Furthermore, you cannot change the role yourself.
In the WebApp: To change a role, you must navigate to the "Employees" subpage in the web application. Then click on the employee whose role is to be changed. A pop-up menu appears in which a new role can be selected via dropdown.
In the mobile app, navigate to the department in which the employee is located. Tap on the corresponding name to open the employee's menu. The role can be changed there.