If you have forgotten to create a reservation, or need to modify a reservation for a certain date in the past, you can still create or modify the booking in Mews.
To do so, you must have your Editable history windows open for this date in the past.
You have 2 Editable history windows: Operational and Accounting. The Operational one is located under Menu > Settings > Property.
The Accounting history window is located under Menu > Settings > Property > Accounting configuration.
7 days in the past is the maximum for both history windows. For reservations, the window starts counting at the default check-in time you have set in your settings.