HVAC problem, no heated air flow?

The OE method for this repair is basically to remove the dash, discharge the AC and drain the coolant, remove the HVAC case and split it so you can access the door. Then replace the heater core and AC core (unless they are in very good condition) and reverse the process to put it back together the book time is over 12 hours. Or you can do a shorter method that involves far less work to do. It is cutting the side of the HVAC case open, replacing the door and then you glue the cut section back in. There is an excellent write up using an equinox (same HVAC case) at:

https://www.terrainforum.net/threads/2010-2016-hvac-blend-door-replacement-without-removing-the-dashboard-hack.28713/

With a pdf download that has images of the process. The cuts can be done far neater if you have a hot knife or one of the vibrating tools.
I use the same process myself as it cuts over 10 hours off the book time and you can still drive the vehicle if needed because the interior is still together.