In most cases, a blockchain is managed by a peer-to-peer network. All peers use a common protocol that specifies how they should communicate with each other, how a new block is created and validated. Once recorded, the data in any given block cannot be changed easily any more. Changing the block means all the blocks after it need to be changed as well. Depending on the protocol, this will require a majority of the peers, or even all the peers, to agree.