I want to change my reservation from three nights to five.
Literal
Reservation [object-を] 3-nights from 5-nights to change-want.
A blunt plain-form version of a hotel-counter request. Without softening like ~のですが, this register fits a journal entry or talking to oneself, but you'd never say this to a hotel clerk directly. The counter 泊 (はく) counts overnight stays, and から…に ('from…to') marks the change.