You don’t have to accept any offer that you don’t want. If you have changed your mind about your firm choice, then there is no reason to blindly accept it, especially if you want to study something else entirely. You will need to either have no offers at all or you will need to have rejected all made to you before you can enter Clearing. Do not feel bad. You can always find different universities or courses that will better suit you and your plans moving forward, so don’t feel you have to accept if you don’t want to.