@Leonie138 I don't know what you mean by you "unmarked" the room she didn't want and "marked" the one she wanted, but "change reservation" only pertains to the listing that was booked, it can't be used to change a guest to another unit.
What needed to happen is she needed to cancel the first booking and then book the other room. But if she does that now, the payments will be all screwed up.
You can't unblock the days because there are now 2 active bookings for both rooms. You can't sort this out yourself, as Helen says, you'll need to contact customer service.
And your listings are classified incorrectly. They should be listed as private rooms, not private studios. A studio means it's self-contained and has a kitchenette or some cooking facilities. But your rooms share a kitchen, and other common spaces, so they aren't studios nor entire homes, as they are currently listed.