Where is the thermostat located and how do you replace it located on a 1998 Nissan Sentra?

The thermostat is located in the water outlet which is attached on the lower end of the top radiator hose. There will be 2-3 bolts holding it in place.Remove them carefully ,tap on housing to remove, and thermostat is within. You don't need to drain down the system any more the the level of the stat. Dry and clean the water outlet mating surfaces, install the new stat in the same way old one was removed, If a gasket is used instead of a o-ring use a gasket sealer of your choice on the gasket. Tighen the 2-3 bolts snugly and evenly to avoid cracking the housing. Fill with coolant and leave rad cap off for 15 minutes to bleed air out and top off coolant. I hope this helps you. Mark