Shanghai Upgrade or Shapella is a hard fork (a big change to Ethereum network's protocol) that was activated on 13th April 2023. Shanghai Upgrade fully switched Ethereum to a Proof of Stake network. Earlier in September 2022, Ethereum moved from Proof of Work (PoW) to a Proof of Stake (PoS) consensus mechanism, however, users were not able to withdraw their staked ether. After Shanghai Update, now users can also withdraw their staked ether.