@Robin First off, the link in that help article is un-borked now This is the article you want. It goes into detail about how you would modify the terms of a reservation.
But I also want to address your specific use case. If you only want to adjust a night, but don't want to adjust the price, follow the instructions in the article above, change the stay dates on the first tab (Stay Details), and then on the second tab (Rental Terms) we give you the ability to quickly remove any change in price (the change in price happens because usually changing the length of the stay automatically changes the rent and/or fees):
@Rikke-Page No problem - glad you were able to make the change.
Side note: having you build a new quote to change the reservation terms puts you in a position to have a very clear historical view of how your reservation has changed - for bookkeeping, problem solving, etc. If you modify the reservation terms, you can go to the Timeline from the reservation detail view and click View Modification.
I'll add one more thing - if the booking has already been made, you can also refund back 10% post-booking (per your question on that). Just do a partial refund against the payment on the payment tab for that Reservation.
@Carole - how would you handle those rate requirements on your listing sites such as HomeAway? If you're using channel management, and the receiving listing site doesn't support that pricing structure, you'd have an issue. Where are you seeing the ability to set that kind of rate on a listing site?