How to change thermostat 2008 Kia Rio.Heater temp. is poor,could thermostat be stuck or defective?

Have you tried running it with the thermostat removed ?
Could be a failed water pump or a leaking head gasket among other things.

This could be one of a few options.#1 could be as little as 1 quart to one half gallon low on coolant in radiator; #2 could be thermostat may be stuck in Open position all the time ( check temp gauge to see if engine is at Normal operating temperature ) #3 could also be stopped up Heater Core....Check this by simply touching heater hose going into heater core and heater hose exiting heater core, at any rate if the core isn't stopped up, Both hoses Should be warm or hot at regular operating temperature of engine warmed up.

Possible causes include low coolent level ( I know it sounds wrong, but if the coolent level is low hot coolent won't circulate to the heater core)

The thermostat may be stuck open, if it is the engine won't reach proper operating temp. Only solution is to replace, but it is easy to do and thermostats are inexpensive.

Heater core may be plugged, this is more likely if stopleak has ever been used to stop a radiator leak. Solution would be backflushing the heater core.

The valve that controls the flow of coolent to the heater core could be stuck or defective or the vacuum switch that controls the valve might be defective.

sounds like you have a bad heater core leaking or seaping radiator fluid into the air ducts

is the cooling system full,if so you may hve to get the heater core cleaned as they can plug up if the dexcool anti freeze has not been changed,could also have defective thermostat(stuck open)if you have gauge does it go half way or stay low temp,other causes could be stuck air blend door in heater system

have to change seat belt

Replace the thermostat and probably radiator.

Intake air temp sender or coolant temp sensor.

First thing to test would be the thermostat. If this is stuck open the engine would never reach correct temperature.

the themostat sounds to be stuck open , take your vech in and have the coolant system flushed and the themostat changed have them check your belt too .