I'd like to change my reservation from three nights to five nights.
Literal
Reservation [object-を] 3-nights from 5-nights to change-want-[explanatory-のですが].
The counter 泊 (はく / ぱく) counts overnight stays — 三泊 'three nights,' 五泊 'five nights.' から...に ('from...to') is the directional pair for changes of state, direction, or amount. This is textbook hotel-desk register.