Reduced the duration of stay

Hello, I need to know how can I modify a booking where guests book for more than 1 nights and stay only one night and leave. 
I feel this must be something that booking.com should help their partners with. As we have to unnecessarily pay the commission even for the days customers didn’t stay. 
So I had a reservation today for 2 nights but they had to leave in 1 day due to personal reasons and we did refund them the cancellation amount and now I cannot modify their booking as they only stayed one day. The only option I see right now is to make entire reservation as “no-show” which isn’t useful for me as I need to modify the duration of stay for which I should pay the commission.