How do you level the floor on a house built in the 40s if it seems to be too low in the middle of the house?

It depends on what is causing the floor to be low. Rot, bad construction, overloading, shift in foundation. Without knowing the cause it is hard to know the remedy, Simple answer is jack it back up with 1, 2, 3, support posts under a beam that goes across the affected floor joists. If you do not have a fair amount of knowledge of construction, I would sugest that you have a professional do it. To just jack up one section of the floor/house, it should not be overly expensive.