I would of accepted a uni by then but would it be possible to cancel it for a clearing when that day happens?
If you are holding an offer from a previous cycle then that uni needs to release you from it. Once they've done that, then you'd need to pay an application fee for this cycle.
If your offer is from the current cycle, then from results day you'll have a "release me into clearing" button on Hub. So you can do it yourself immediately.