How do I modify the price for one night for an existing reservation?

To modify the rate for a given night in a reservation, open the reservation and scroll down to the invoice items. Delete the item for the night you wish to modify (click the 'x' button). Once you have deleted the item for this night, add a new item with the same name (date name), the udpated price, and select 'Room' for the item type from the dropdown menu on the left.

The new item will be added to the invoice as the price for the given night, with the updated price. Note that if you reschedule this booking, this update will be overwritten when prices are recomputed.