You cannot renew your visa to work in the UK. You were a student and when your course ends you have to return to your home country. If you wish to work in the Uk your employer has to apply for a work permit on your behalf. You cannot apply to work in the UK. In any case if your employer does get a work permit for you you will have to return to your home country to get an entry clearance to start the work on the work permit. Switching from a student to a working permit is not allowed when in the UK. If you start work or remain after your course you will be illegal in the Uk and a knock on the door could result in you spending a few nights in police cells.