The heater of your Nissan produces warm air that is blown into the cabin if the heating is switched on. Warm coolant is injected into the heater motor and blown into the cabin area via the cabin suction fan.
In general a heater motor needs replacing due to leaking radiator hoses. You will easily detect any leaks as you'll find white spots in the vehicle's cabin.
Replacing a heater motor is a rather complicated reparation and unless you have experience changing car parts and are well-equipped, it is better to ask a professional mechanic to step in.
To find out which heater motor is compatible with your Nissan, please select the model and engine as well.