Respond via UCAS. Once you have received responses from all of your choices, UCAS will ask you to decide which one you want to accept as a Firm choice and which you want to accept as your Insurance choice. UCAS will provide you information on when your deadline is.
If you firmly accept a university and meet the terms of their offer, that will be the university you will go to. If you do not meet the conditions of your offer exactly of your Firm choice then they will make the decision if they do wish to accept you with the qualifications you did achieve. If your Firm choice university cannot accept you then you will then be considered by your Insurance choice, therefore, you should try to ensure when choosing universities that you could meet your insurance offer if you do not make your firm offer.
Undergraduate international students who have applied through an official agent representative can ask them to accept on their behalf.