If you need a visa extension then you should come to the International Office and speak to a member of the Visas and Compliance team. You should also visit http://www.beds.ac.uk/international/international-student-support/visas-and-immigration/tier4adult/timeframe . You will need to apply for a new CAS online 8 weeks before your current visa expires and the Visa and Compliance Team will have to do a full assessment of your application for a CAS before they can decide whether a CAS can be issued.