As others have said, UCAS will update and then you will be sent an email informing you there has been a change.
With my St Andrews offer, I actually got an email about a week before track updated saying that I would "more than likely" be getting an offer in the near future, they just had to finalise some things (new course route), and after my offer was made official I got another email from them detailing what to do next. So you'll probably get a mixture of emails from UCAS and emails from the uni.