changeing a heater core on 1990 Chevrolet G Series Van (G10)

I would like directions on how to change out a heater core

I think that Bob has a good explanation.
1 more answer
You need to drain the coolant first. Remove the engine cover and air ducts. You'll need to lower the steering column, disconnect many items including the radio bracket screws and the heater control cables, etc. Then detach the dash by removing the upper and lower fastening screws, but don't remove it completely, just enough to remove the heat/vent/A-C enclosure and remove the heater core.
Reverse the procedure and fill with coolant. This is kind of an awkward job but not too hard to do if you have lots of patience.
If you want to find a shop, here's a directory link for you: