Once a transaction is added as a block, it is permanently part of the blockchain and cannot be deleted or changed. Each node on the network stores copies of the blockchain’s blocks on their local resources, creating hundreds of thousands of simultaneous backups of each transaction’s contents. This is so that if any block is presented to the blockchain, it is cross-referenced across the entire network and compared to the history stored on all the nodes, providing the network the ability to spot fake blocks that might be presented to the blockchain in an attempt to steal or hack information or crypto. The only way to truly alter any information on a blockchain would be through compromising and editing at least 51% of the nodes on the blockchain at the same time, which would require an immense amount of money, time, resources, and skill from the bad actor.