To change the price of a Service within a Reservation, locate the Reservation on the Bookings screen and tap on it

If the Reservation only has one Service in it, the Price can be changed in the Price field

Enter a new figure and tap Done

If the Reservation has multiple Services in it, the Price can be changed for each Service separately

In this case, tap on the Reservation, then scroll down to Services list. Tap on each service individually, make the necessary edits and tap Save