= Solution
Define $\Pi^\mu{}_{\nu}=\operatorname{diag}(1,-1,-1,-1)$ and $x_P=(x^0,-\mathbf x)$. In a conventional intrinsic-phase choice, the charged <W boson> transforms under <CP symmetry> as
$$
\boxed{W^{+\mu}(x)\longmapsto-\Pi^\mu{}_{\nu}W^{-\nu}(x_P).}
$$
Thus its time component maps to minus the conjugate time component, and its spatial components map to plus the conjugate spatial components. Charge is exchanged, the Lorentz index undergoes <parity>, and the overall minus sign is a consistent phase convention shared with the charged-current transformation. The analogous formula exchanges $W^-$ with $W^+$.
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