Editing the dates

I have had some issues more than once about editing a booking. For example I am in the process of completing a boarding with a dog and the pet parents are changing the booking from 7 night to 6 nights, I am having trouble editing the booking and changing the dates, I should be able to do that on my end. Now what’s the best thing for me to do now? Because I can’t charge her for that extra night she’s now not using.