You can cancel future annual leave yourself if none of the dates of the leave have passed. If you need to cancel annual leave that occurred in the past, your manager should raise a case to instruct the NSSO to cancel the leave.
If your manager approved a half day of leave and you now wish to take the full day, the system will not allow you to apply for a second half day on the same date. If the day is in the future, you can cancel the half day and apply for a full day. If the day is in the past, your manager must raise a case instructing the NSSO to cancel the half day and replace it with a full day.
Please note: If you have been out of work due to illness or injury, this is an unscheduled absence from work and not leave. You can find information and guidelines on sick leave absences in the unscheduled absence page of this site.