This question is a simplified version of why the blockchain is secured. Say all the miners are working on block 91 of the chain and someone wants to alter a transaction in block 74 then that miner will have to make all the changes while re-doing all the computations from block 74-90 and then do block 91. Not only that the 18 blocks is expensive to compute as it will need a very powerful computational power but everything would have to be done before everyone else in the Bitcoin network finish working on block 91.
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