The whole point of a democracy is that we can change our minds. We have General Elections. We can change the Government.

In Ancient Athens when they had a direct democracy they originally voted to execute all the inhabitants of the island of Mytilene. Later realising they had been stupid, they had another vote and decided not to execute all the inhabitants.

Another point is that some of the people who voted to leave have probably died and we must have a lot of people who could not vote back in 2016, who now have the vote.