The magnitude of the normal force will decrease when force F is directed upwards.
Work Step by Step
The normal force is the force from the table to the block, that is opposite the force of gravity. This force is equal and opposite to the force of gravity (in this situation), so increasing the force of gravity effectively increases the normal force. Applying force in the same direction as gravity (downwards) will act like the force of gravity is increasing, and the normal force will increase. In this situation, force is being applied in the opposite direction so the normal force will decrease.