How do I restore the thermostat to its original factory settings?

You can restore original settings using system setup Function 90. Set Function 90 to '1' to disconnect the thermostat from Wi-Fi and restore original settings, which will erase all customizations. To enter system setup, press Fan and the up arrow simultaneously and hold for approximately 3 seconds. (Pages 48–49, 54)

From the official Honeywell RTH6580WF Wi-Fi Smart Thermostat manual, page 54

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