You are unable to change your student username. Your username is a unique identifier for University systems as well as identifying you in information links between the University and the Ministry of Education (e.g. your student index number) and WINZ. Any changes to your username could possibly affect these links and cause problems now and in the future.
Your username is generated at the time of your first enrolment and is retained in order to ensure continuity over the years with your changing status, including any changes of name.