Boolean difference removes wrong part I have a cube that is scaled about 10 times on the Z axis.
I added a plane, rotated it 45 degrees on the Y axis, and placed it just below the top of the cube. I applied a boolean difference modifier to the cube, and it cut off the bottom. How can I invert the boolean operation to cut off the top of the cube instead? The plane should be deleted after the cutting.
The Boolean modifier calculations depends on the face normals to decide what's inside and outside ; for a simple plane case the face normal is the local z_axis so it's enough to flip the plane 180° 

for other case :


enter edit mode hit Ctrl+N 
check inside option on the tools panel