How do you go about replaceing a evaporator core in a 1997 GMC Yukon SLE ? on 1997 GMC Yukon

It does not blow any cool air at all. the vent door will not close when going from heat to air conditioner

Asked by for the 1997 GMC Yukon
if the blend door isnt working then I wouldnt worry about the evap core and fix the door