Once data is written to a block, its difficult to modify it. To modify the data in one block, we need to modify all subsequent blocks data to maintain hash associated with a block.
Block in the blockchain are immutable (cannot modify). It will always create a new block whenever someone need to update the block. They are linked with hash and designed as linked list, where one block has information about its previous block. First block is called Genesis block (which does not contain previous block hash.)