If a hacker starts changing records in one ledger, all other nodes would reject it, since new record will not match the stored data. There’s no centralized authority that can manipulate a Blockchain. The only way to manipulate data is for every single node to conspire together, which is unlikely. In the case of Bitcoin, there are roughly 10,000 different Bitcoin nodes spread across the world. Some blockchains are susceptible to 51 percent attacks, whereby, a group of attackers controls more than half of a blockchain’s computing power. For large networks, like Bitcoin’s blockchain, that’s very unlikely to happen, but it still remains a threat for smaller blockchains with fewer miners.