Very first thing to do is ensure the coolant level is up to the mark and if low top it up with the correct antifreeze or pre-mixed coolant as stated in the owners manual
I'd then check that the engine is getting up to temperature, - are the radiator hoses hot and the temp gauge reaching the middle position?
If this is OK have a feel of the two coolant pipes that pass into the car through the engine bay bulkhead, both should be warm with a warm engine. If they're not then its likely the heater matrix is blocked and you should do a reverse flush on the cooling system to try and clean it out. This is done by removing the top hose from the radiator and placing a garden hose into the engine water outlet, and then run the hose until you get clean water coming out of the radiator.
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