When to replace? Its simple. Replace it when its broken.
How to replace? Follow the following steps:
Before replacing it, remove the negative terminal of the battery
At first, raise your vehicle from the front
Then you need to drain the coolant and antifreeze from the radiator
Now remove the hoses from the heater core
You can lower your car back to the normal position.
Now locate the heater cores and remove the mounting clamps of the same.
Remove the heater core and dry the carpeting by side.
You can now install the new heater core as per guided and suggested by your dealer or as per mentioned in the car manual.