1987 cadillac fleetwood brougham delegance.

How do you replace the heater core?

I did a really good coolant flush, and changed the thermostat.
But the heater core may be plugged. I want to replace it.
When at 90 degrees F and full fan, the driver side blows hot but the passenger side blows not as hot.

I also want to vacuum and clean any possible debris.