You will have to drop out and reapply, as you cannot transfer to a different course part way through the year. The first step is to speak to your personal tutor, and find out what the financial implications of dropping out would be. If you don't live in university accomadation, you need to discuss this with someone from your accomadation too. You would then have to reapply through UCAS as you did last year. Do bear in mind that maths courses are not keen on gap years, so it might be worth phoning up admissions departments to discuss your situation.
You need to think very carefully about your decision though. You can drop out once, and still get funding for the full length of a course from SFE. However, you would not be funded to repeat a year, or able to get full funding if you drop out again.
If you plan to move back home, you should also discuss this with your parents.