Why would heat turn to cold when you slow down on a 2002 Nissan sentra?

Check the radiator coolant level! When it is low, and RPMs decrease at low speeds or idle, the coolant isn't being circulated through the entire cooling system, such as the heater core! Be sure the engine is cool before opening the radiator cap and adding coolant! NEVER add coolant to a hot engine because it will crack the block or cause the head gasket to blow!