0x3659cfe6?0x3659cfe6 is the function selector for upgradeTo(address). Replaces the contract logic with a different implementation.
When a transaction's data begins with 0x3659cfe6, it is calling upgradeTo. The selector is the first four bytes of the keccak-256 hash of the signature upgradeTo(address), which is how wallets and explorers work out which function a payload is for.