What is the net force on an apple that weighs 1 n when you hold it at rest above your head?

the net force on the apple is zero.

Explanation:

The Law of action and reaction, which reads: "Every action corresponds to an equal reaction but in the opposite direction: it means that the mutual actions of two bodies are always the same and directed in the opposite direction." Which means that for each force exerted on an object, a similar one exerted by it is opposed, in the opposite direction and of equal intensity.

As in this case there are 2 opposing forces whose sum is zero. One is the force of gravity in a downward vertical line and the other is that of supporting your body by having it on the head that exerts an upward force. These forces are opposite and equal making the net force zero.