They open the same link and change it. The invitation remembers them, so the page they come back to shows what they said and a Change control beside it.
The count on your event page moves the moment they do, and the change is in their history on their row, so you can see they said yes on Tuesday and no on Friday rather than wondering whether you misread it.
You can also change an answer yourself. Open the person's row and mark them yes or no — useful when someone tells you in person, or at the door. The row records that you were the one who set it.
There is no deadline on changing an answer. People's plans move, and an invitation that punishes them for saying so gets fewer honest answers.