When using a micropipette, the volume of liquid aspirated is determined by the position to which the plunger is depressed before aspirating. If you depress the plunger to the incorrect position (which is typically beyond the first stop, where the liquid is meant to be aspirated), you will not create the proper vacuum to draw up the correct volume of liquid.
If you instead pull up the plunger to aspirate after pressing it to the incorrect position, less liquid will be drawn in because you have already displaced a greater volume by pressing too far down.
Therefore, the correct answer is:
Volume decreases.