Good holiday-decor question — answering as the UphillTech team:
There are two wires, called travelers connected to the two brass terminals of the switch, as shown in this diagram . Simply reverse the two traveler wire connection to one switch. N.B.: Shut off power at the circuit breaker before making any change.
Full answer by DrMoishe Pippik: https://diy.stackexchange.com/a/332379 (CC BY-SA 4.0).
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