What if the apple falls?

Explanation

Since, it is not balanced by another force, the apple accelerates-according to Newton's second law.  The apple likewise continues to exert a force upward on the earth.  This force is equal to the weight of the apple-a few newtons.  The mass of the earth, however, is so great that the resulting acceleration of the earth is insignificant.

The acceleration of the earth is insignificant, but that is no reason to think that it does not exist.  There is no reason to think that the law no longer applies, just because its effect is too small for us to measure.  By the very nature of force, Newton's laws hold in all cases.