But that doesn't answer the question of how the people can tell parliament that it changed its mind...
If opinion polls and/or common sense observations, showed a major swing, in a particular direction.
E.g. If the EU made major changes and announcements, which significantly changed the landscape. E.g. changes to how "free movement" works, etc etc.
The government should/would be able to spin things, such that a general election (with one side offering a second referendum) and/or second referendum takes place.
52% vs 48% is pretty close, and two years+ is a long time (in some respects).
The second referendum could be something like:
(A)...Deal A, and Leave
(B)...Deal B, and Leave
(C)...Remain and accept compromise deal C
But in practice, the decisions been made, and changing it would be rather difficult.
Also once article 50 has been activated. As far as I know, there is NO mechanism for reversing it. Except rejoining, at some later date. Which could take a VERY long time.
There could also be legal challenges, if there were attempts to change it. Since the vote has already occurred, it is too late.
I suppose one solution, would be for parliament to vote on if to have a second referendum or not. Since I think they need to vote, in order to (legally) activate article 50, anyway. If I remember correctly, September 4th 2016, they are debating having a second referendum, anyway, since it has been requested by lots of people, online.