Answer
Undergraduates should talk to their College tutors, who will advise them on the best course of action.
Graduates need to fill in a Graduate Studies Office (GSO) form, available here. You will need to have the form signed by your Supervisor before emailing itĀ to the Academic Office for approval by the Tutor for Graduates. Remember to submit the form to theĀ Academic Office two or three days before the deadline, as it may not be possible for it to be signed immediately.