Coolant flush

More than likely you are not getting the coolant out of the block and heater core. However, this is not completely necessary but if you want to completely flush it out, you will most likely need to rent a power flush machine from a parts store.

Here are a couple of guides that will help with doing this process:

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

https://www.youtube.com/watch?v=iuEVLFEF0AM

However, I would suggest just doing it the old school way which is just adding distilled water in the radiator and run the engine until the coolant runs clean. You have to remove the thermostat to do this.

You will want to route the upper hose from the thermostat to a bucket so that you don't make a complete mess doing this.

https://www.2carpros.com/articles/replace-thermostat

Then all you need to do is hook everything back up, install the thermostat, make sure the radiator is empty and fill it back up with full strength coolant. This will mix with the distilled water that is still in the engine and heater core and be the correct mixture.

This method will work but if you can get a machine to flush it, then that is much better.

Lastly, you do not have to flush it all if you are just doing this as normal maintenance. If you have no issue with the coolant but just changing it, you will be just fine draining what you can get out and then filling with new coolant.