how do you replace a thermostat in 2500 express van 1998

Pretty easy; Find where the upper hose goes to the intake at your thermostat housing. While taking the nuts and bolts out use caution not to break the grounds. Remove the housing, and look at the thermostat and what direction the spring is. Clean the mating surfaces, install the thermostat the same direction as the old one was. Then seal and tighten, and refill the coolant system.