Mastering Blockchain Programming with Solidity

Irreversible transactions

Once the smart contracts are deployed on Ethereum network, their code becomes immutable. No one can change the code of the deployed smart contract. It's the same as when some transaction on the Ethereum network is done, and there is no way to reverse that transaction. For example, if person A sends 1 ETH to person B and the transaction is successfully executed, then there is no way for person A to reverse that transaction and to get that 1 ETH sent back to their wallet. 

For smart contracts as well, if you have done a particular transaction on a smart contract that has changed the state of the contract, then there is no way to reverse that transaction. There might be some other functions available in the smart contract ...