Are smart contracts immutable?

Smart contracts on the Ethereum blockchain are immutable, which means that once a smart contract is deployed on the Ethereum blockchain network, the code cannot be changed by anyone.

Specifically, this immutability characteristic does not allow for any modifications and/or reversal subsequent to the deployment of the smart contract to the blockchain.

However Smart Contracts can hold data which, depending on how they are programmed, can be changed.

This is true even for immutable contracts. It is this facility which allows for the construction of Upgradeable Proxy Contracts.