Earlier quoted context omitted.
The logic exists on the block chain. It is executed on each and every machine that follows the chain. This is how executions are verified, 'everyone' has to agree on the outcome.
Sorry for being so uninformed but what happens if there is an error in the logic? Doesn't that mean that it's not possible to update the smart contract?
If you are a big name in the Ethereum community, they will soft fork Ethereum so that your mistake is rolled back.