Once an Ethereum smart contract is deployed, its code cannot be changed. This is achieved through the EVM (Ethereum Virtual Machine) and the nature of blockchain itself.

Why?

• Security: Immutability ensures that the contract's logic cannot be altered maliciously, making it resistant to tampering.
• Transparency: The contract code is accessible by anyone, which allows for full transparency and trust in its behavior.
• Auditability: Code that cannot be changed is easier to audit, as there’s no concern of hidden updates or alterations.

Bad things

If a contract is deployed with a vulnerability, you cannot directly fix the contract. The only option would be to deploy a new version, which might not be practical if the contract holds valuable assets or has been in use for a long time.

Ref - https://nftnow.com/features/akutars-exploit-34-million-locked-in-smart-contract/

https://x.com/0xInuarashi/status/1517674505975394304