How to replace a heater core?

What is your question?

I just bought this car and the heater and ac don't work. There is a funny smell that fills the car when I'm driving. I was informed that there is an issue with the heater core and I'm pretty sure that means it's leaking right? I'm wondering if I should replace it or just buy some K-Seal or something similar. How do you replace a heater core in a 96 ford aspire? Any advice?