If you have completed all your studies and only have some resits to do, then it will not be possible to extend your visa in the UK. Instead, you will need to return to your home country and make an application to return to the UK on a Student Visitor Visa. Links to the Student Visitor Visa application form and guidance notes are available here:
http://www3.imperial.ac.uk/international/current/visas/applicationforms
The College Registry can produce a Student Visitor Visa letter for you with details of the month when your resit examination will be held.
