Recently Firo, former Zcoin had a 51% attack and it was possible to change/tamper data of 306 blocks(or so). What exactly is that? Is it like the guy who had control of 51% of the network had overwritten the block? How does anyone get benefitted from such an attack? What's the step to prevent such attack?
https://cointelegraph.com/news/privacy-focused-firo-cryptocurrency-suffers-51-attack