You can do this using $CNOT$ gates with the first qubit as control. This will keep the term $\alpha \ket{000}$ unchanged and flip $\beta \ket{100}$ to $\beta \ket{111}$.

@[solution]({
    "id": "qec_shor__bitflip_encode_solution",
    "codePath": "Solution.qs"
})
