Do you mean changing course within the same uni? Your first point of call should be an advice centre or something at your uni - they'll arrange an appointment for you with an adviser, and just explain why you'd like to change and they may be able to see if they can sort something out internally for you, giving you advice on accommodation/fees matters too, of course. But otherwise, you'd just need to withdraw and start a new UCAS application... obviously if you want re-entry for 2011 you'd be classed as a late applicant, so there would be a limited number of places at universities available. Might be worth contacting the unis you're interested in individually by phone and seeing what they say.