Changing prices in an existing booking is sometimes needed, preferably only in manual bookings or own website bookings. (Be aware of the consequences when daily prices are changed for OTA bookings as this is usually not desired.)
On the booking page of the concerning booking you would like to change prices for go to the "Room/Allocation" section and click on the more button (eye symbol) :
In the pop up screen that appears click on edit (pencil symbol)
You are now able to change the price under the rate plan column.
Hit Save to finalise the change.: