You can bring forward assessment for up to two subjects per academic year. If you want to sit an examination early for a subject running between June and February, you need to request this in December. If you want to sit an examination early for a subject running between September and June, you need to request this in April. The essential requirement in both instances is for you to have enrolled on the subject at least once and that the lecturer agrees it can be re-taken (see the corresponding course guide).