the easiest way to change a unit of study is to do it through myuni - you access this by the unikey account you would of been given at enrolment. just log into myuni and cick on 'enrolment variation' which is on the left of the main screen under 'quick links'. to change you will need to know the course code of the new subject you want - and be sure to dis-enrol out of the other one or you will get charged for both and an absent fail.
I think you can add a unit of study up until about week 2, and last time to withdraw from one is some time at the end of march...