I would like to change the filter on my Delco Conditionair. The unit is in the attic, which is accessed by a trap door in the ceiling. Using a step ladder and sticking my head through the opening, I am unable to see much other than the name and a phone number for the service that installed it, which starts with TX 6!

Well, you will need to get into the attic and find the filter slot.

Where might it be? On the top? I cannot get into the attic.

They are usually in the air handler. However, look for a large square cold air return in the first floor wall, my filter is in there. If it is about 20X20 or 24, your filter is in there. Release the clips, open the front and remove the old filter. Get a same size replacement and make sure the air flow arrows are going the right way. If not, up you go!