History is written by the victors is a truism that seems obvious, and it is true to some extent; but what is more relevant is that history simply relates to the point of view of the historian. There are facts about history that are indisputable: Napoleon was crowned Emperor of France on Dec 2nd 1804, Columbus landed in the Americas in 1492 and Stalin was born on December 18, 1878. But many facts of history are drawn from lots and lots of little indisputable facts, and are really opinions based on people's actions, thoughts and aims which can sometimes only be surmised in hindsight: were British generals really donkeys leading lions in WW1 for instance? Or was the Soviet State destined to collapse economically due to structural inflexibility or was it just simple mismanagement? Sometimes historical opinions are not really based on any specific facts but actually based on just-so hypotheses, prejudices and wishful thinking, e.g.: Neanderthal men were brutal wife-battering club-carrying idiots, likely called Ug.
Historical revisionism is the process of historians re-examining some of these prejudices and wishful thinking errors that have built up over time. Of course, even modern historians fall foul of those same biases, so it requires process that can mitigate these problems. Science - reliance on evidence, peer review etc - gives a rule-book and a way out of the tangle. Unfortunately, there is also pseudoscience in history too. Holocaust denial is a prime example. Deniers use the excuse that they're doing historical revisionism when they excuse, minimise or simply refute the fact that 6m Jews were systematically murdered by the Nazi state. Sometimes the revision can be state-sponsored: in The Soviet Union, Stalin ordered changes in photographs and textbooks to erase from history those he deemed as traitors, which ultimately distorted the learning process in Soviet educational institutions.
But just a science changes its conclusions based on new evidence, so must history. It also important that we revise conclusions based on better understandings as well as new facts. In what sense did Europeans 'Discover' the rest of the world? And why should history be so focused on great (white) men? History is constantly being written as soon as events happen. One wonders what historians will think of the Brexit process if (or when) it all goes badly: did plucky Britain stand alone against the mighty EU bureaucrats but ultimately fall due to the Europeans' treachery, or will they look back and think it was an ignominious period mismanaged by incompetent politicians which led to our diminishment to a second rate country?