Change price in confirmed reservation


There has been a change in one of my bookings which affects the final price of the stay.
Eventually there will be fewer guests and it would be necessary to change the number of people confirmed so that the local tax is the one that really applies.

On the other hand, the customer asks me if I can lower the price and I would agree to make a small discount as they will be 2 people less traveling. Is there any possibility of making this modification once the reservation has already been confirmed? Thanks for your help