0x2a6a833b?0x2a6a833b is the function selector for upgradeAndCall(address,bytes). Replaces the implementation and calls into it.
When a transaction's data begins with 0x2a6a833b, it is calling upgradeAndCall. The selector is the first four bytes of the keccak-256 hash of the signature upgradeAndCall(address,bytes), which is how wallets and explorers work out which function a payload is for.