guv1976 has it right. Under the current rules, cancelling the return after flying the outbound will net you a refund of the miles for that leg. So you might as well do this instead of no-showing for the return. You’re not able to modify the reservation before flying the outbound. You could cancel the round trip and rebook as a one way, but it really only makes sense to do that if the one-way would be cheaper than the outbound portion of the round trip you already have.