Lennox G40UH(X) series - blowing cold air outside - would like to know what I can check before having to spend money for service to come check it? Can I check filters, fans, wires, etc?

You are having a problem with the reversing valve. It could be a stuck reversing valve or an electrical problem. If you need a new reversing valve it will be expensive because the gas will have to be removed and a new reversing valve welded in. If the unit is old it would be best to just replace it. DrTom4444

Could it be that wires are plugged in the wrong place on the unit outside? I compared the wire connections between both units and noticed the orange wire on the 1st unit was different than the 2nd unit (which is not working properly). I moved the orange wire on the 2nd unit to match that of the first unit. Not sure if this will work or not. Could it be that upstairs takes long time to cool down and I just need to be patient? Could it be that the 2nd unit is working too hard causing something to stop working on the unit itself? Last question, should I check the furnace in the attic?

Did it just start doing this? The orange wire controls the reversing valve in most units, otherwise the blue wire controls it in some latitudes. It all depends on what area you are in: some heat pumps are designed to fail in heat mode and some in ac mode. In Florida the Orange wire must be energized to get AC and in some states the blue wire must be energized for heat. You need to ohm out the solenoid on the reversing valve and find out if it has 24 volts because to get ac it should get 24 volts. DrTom4444