How do you replace thermostat for a 1988 Dodge Aries Also would the thermostat cause both AC and Heater not to work properly?

Drain coolant. Follow top large rad hose down to engine; where it meets engine you will find the thermostat housing probably connected to the motor with two bolts; remove them. Gentley pry up housing; note inside of houing is stat; note position, now remove stat and relace with new one. install new gasket with a silicone sealer; carefully tighten down both bolts equally; snug up only do not over tighten. Install hose. Fill with coolant; keep rad cap off for 20 minutes to bleed out trapped air. Done. A stat does not affect AC; only heating and cooling.